Golf ball

ABSTRACT

A golf ball  2  includes a core  4 , an inner cover  8 , a mid cover  10 , and an outer cover  12 . Nine points are determined by dividing a region from a central point of the core  4  to a surface of the core  4  at intervals of 12.5% of a radius of the core  4 . When distances (%) from the central point to the nine points and hardnesses at the nine points are plotted in a graph, R 2  of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95. A hardness Hi of the inner cover  8  is greater than a hardness Hs at the surface of the core  4 . A hardness Ho of the outer cover  12  is greater than the hardness Hi. A difference (Hi−Hs) is 1 or greater. A difference (Ho−Hi) is 5 or greater but 30 or less.

This application claims priority on Patent Application No. 2012-119167 filed in JAPAN on May 25, 2012, Patent Application No. 2012-119217 filed in JAPAN on May 25, 2012, Patent Application No. 2012-121710 filed in JAPAN on May 29, 2012, and Patent Application No. 2012-121943 filed in JAPAN on May 29, 2012. The entire contents of these Japanese Patent Applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to golf balls that include a solid core and a cover having a plurality of layers.

2. Description of the Related Art

Golf players' foremost requirement for golf balls is flight performance. In particular, golf players place importance on flight performance upon shots with a driver and a long iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball having excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance. Golf balls that include a core having excellent resilience performance are disclosed in JP61-37178, JP2008-212681 (US2008/0214324), JP2008-523952 (US2006/0135287 and US2007/0173607), and JP2009-119256 (US2009/0124757).

The core disclosed in JP61-37178 is obtained from a rubber composition that includes a co-crosslinking agent and a crosslinking activator. This publication discloses palmitic acid, stearic acid, and myristic acid as the crosslinking activator.

The core disclosed in JP2008-212681 is obtained from a rubber composition that includes an organic peroxide, a metal salt of an α,β-unsaturated carboxylic acid, and a copper salt of a fatty acid.

The core disclosed in JP2008-523952 is obtained from a rubber composition that includes a metal salt of an unsaturated monocarboxylic acid, a free radical initiator, and a non-conjugated diene monomer.

The core disclosed in JP2009-119256 is obtained from a rubber composition that includes a polybutadiene whose vinyl content is equal to or less than 2%, whose cis 1,4-bond content is equal to or greater than 80%, and which has an active end modified with an alkoxysilane compound.

An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. With a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. With a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of an outer-hard/inner-soft structure in a golf ball can achieve a low spin rate and a high launch angle. Modifications regarding a hardness distribution of a core are disclosed in JP6-154357 (U.S. Pat. No. 5,403,010), JP2008-194471 (U.S. Pat. No. 7,344,455, US2008/0194358, US2008/0194359, and US2008/0214325), and JP2008-194473 (US2008/0194357 and US2008/0312008).

In the core disclosed in JP6-154357, a JIS-C hardness H1 at the central point of the core is 58 to 73, a JIS-C hardness H2 in a region that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point is equal to or greater than 65 but equal to or less than 75, a JIS-C hardness H3 at a point located at a distance of 15 mm from the central point is equal to or greater than 74 but equal to or less than 82, and a JIS-C hardness H4 at the surface of the core is equal to or greater than 76 but equal to or less than 84. The hardness H2 is greater than the hardness H1, the hardness H3 is greater than the hardness H2, and the hardness H4 is equal to or greater than the hardness H3.

In the core disclosed in JP2008-194471, a Shore D hardness at the central point of the core is equal to or greater than 30 but equal to or less than 48, a Shore D hardness at a point located at a distance of 4 mm from the central point is equal to or greater than 34 but equal to or less than 52, a Shore D hardness at a point located at a distance of 8 mm from the central point is equal to or greater than 40 but equal to or less than 58, a Shore D hardness at a point located at a distance of 12 mm from the central point is equal to or greater than 43 but equal to or less than 61, a Shore D hardness in a region that extends over a distance range from equal to or greater than 2 mm to equal to or less than 3 mm from the surface of the core is equal to or greater than 36 but equal to or less than 54, and a Shore D hardness at the surface is equal to or greater than 41 but equal to or less than 59.

In the core disclosed in JP2008-194473, a Shore D hardness at the central point of the core is equal to or greater than 25 but equal to or less than 45, a Shore D hardness in a region that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point is equal to or greater than 39 but equal to or less than 58, a Shore D hardness at a point located at a distance of 15 mm from the central point is equal to or greater than 36 but equal to or less than 55, and a Shore D hardness at the surface of the core is equal to or greater than 55 but equal to or less than 75.

JP2010-253268 (US2010/0273575) discloses a golf ball that includes a core, an envelope layer, a mid layer, and a cover. In the core, the hardness gradually increases from the central point of the core to the surface of the core. The difference between a JIS-C hardness at the surface and a JIS-C hardness at the central point is equal to or greater than 15. The hardness of the cover is greater than the hardness of the mid layer, and the hardness of the mid layer is greater than the hardness of the envelope layer.

For a tee shot on a par-three hole and a second shot on a par-four hole, a middle iron is frequently used. Golf players also desire a large flight distance upon a shot with a middle iron. An object of the present invention is to provide a golf ball that has excellent flight performance upon a shot with a middle iron. Another object of the present invention is to provide a golf ball that has excellent flight performance upon a shot with a middle iron and has excellent feel at impact.

SUMMARY OF THE INVENTION

A golf ball according to the present invention includes a core, an inner cover positioned outside the core, a mid cover positioned outside the inner cover, and an outer cover positioned outside the mid cover. When distances (%) from a central point of the core to nine points and JIS-C hardnesses at the nine points, which nine points are obtained by dividing a region from the central point of the core to a surface of the core at intervals of 12.5% of a radius of the core, are plotted in a graph, R² of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95. A JIS-C hardness Hi of the inner cover is greater than a JIS-C hardness Hs at the surface of the core. A JIS-C hardness Ho of the outer cover is greater than the hardness Hi. In the golf ball according to the present invention, hardness distributions of the core and the inner cover are appropriate. In the golf ball, the energy loss is low when the golf ball is hit with a middle iron. When the golf ball is hit with a middle iron, the spin rate is low. The low spin rate achieves a large flight distance.

According to another aspect, a golf ball according to the present invention includes a core, a first cover positioned outside the core, a second cover positioned outside the first cover, a third cover positioned outside the second cover, and a fourth cover positioned outside the third cover. When distances (%) from a central point of the core to nine points and JIS-C hardnesses at the nine points, which nine points are obtained by dividing a region from the central point of the core to a surface of the core at intervals of 12.5% of a radius of the core, are plotted in a graph, R² of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95. A JIS-C hardness H1 of the first cover is greater than a JIS-C hardness Hs at the surface of the core. A JIS-C hardness H4 of the fourth cover is greater than the JIS-C hardness H1 of the first cover. In the golf ball according to the present invention, hardness distributions of the core and the first cover are appropriate. In the golf ball, the energy loss is low when the golf ball is hit with a middle iron. When the golf ball is hit with a middle iron, the spin rate is low. The low spin rate achieves a large flight distance.

According to still another aspect, a golf ball according to the present invention includes a core, an inner cover positioned outside the core, a mid cover positioned outside the inner cover, and an outer cover positioned outside the mid cover. When distances (%) from a central point of the core to nine points and JIS-C hardnesses at the nine points, which nine points are obtained by dividing a region from the central point of the core to a surface of the core at intervals of 12.5% of a radius of the core, are plotted in a graph, R² of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95. A JIS-C hardness Hs at the surface of the core is greater than a JIS-C hardness Hi of the inner cover. A JIS-C hardness Ho of the outer cover is greater than the JIS-C hardness Hi of the inner cover. In the golf ball according to the present invention, hardness distributions of the core, the inner cover, and the outer cover are appropriate. In the golf ball, the energy loss is low when the golf ball is hit with a middle iron. When the golf ball is hit with a middle iron, the spin rate is low. The low spin rate achieves a large flight distance. Furthermore, the golf ball has excellent feel at impact.

According to still another aspect, a golf ball according to the present invention includes a core, an inner cover positioned outside the core, a first mid cover positioned outside the inner cover, a second mid cover positioned outside the first mid cover, and an outer cover positioned outside the second mid cover. When distances (%) from a central point of the core to nine points and JIS-C hardnesses at the nine points, which nine points are obtained by dividing a region from the central point of the core to a surface of the core at intervals of 12.5% of a radius of the core, are plotted in a graph, R² of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95. A JIS-C hardness Hs at the surface of the core is greater than a JIS-C hardness Hi of the inner cover. A JIS-C hardness Ho of the outer cover is greater than the JIS-C hardness Hi of the inner cover. In the golf ball according to the present invention, hardness distributions of the core, the inner cover, the first mid cover, the second mid cover, and the outer cover are appropriate. In the golf ball, the energy loss is low when the golf ball is hit with a middle iron. When the golf ball is hit with a middle iron, the spin rate is low. The low spin rate achieves a large flight distance. Furthermore, the golf ball has excellent feel at impact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ball according to a first embodiment of the present invention;

FIG. 2 is a line graph showing a hardness distribution of a core of the golf ball in FIG. 1;

FIG. 3 is a partially cutaway cross-sectional view of a golf ball according to a second embodiment of the present invention;

FIG. 4 is a line graph showing a hardness distribution of a core of the golf ball in FIG. 3;

FIG. 5 is a partially cutaway cross-sectional view of a golf ball according to a third embodiment of the present invention;

FIG. 6 is a line graph showing a hardness distribution of a core of the golf ball in FIG. 5;

FIG. 7 is a partially cutaway cross-sectional view of a golf ball according to a fourth embodiment of the present invention; and

FIG. 8 is a line graph showing a hardness distribution of a core of the golf ball in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention, based on preferred embodiments with reference to the accompanying drawings.

First Embodiment

A golf ball 2 shown in FIG. 1 includes a spherical core 4 and a cover 6 covering the core 4. The cover 6 includes an inner cover 8, a mid cover 10 positioned outside the inner cover 8, and an outer cover 12 positioned outside the mid cover 10. On the surface of the outer cover 12, a large number of dimples 14 are formed. Of the surface of the golf ball 2, a part other than the dimples 14 is a land 16. The golf ball 2 includes a paint layer and a mark layer on the external side of the outer cover 12, but these layers are not shown in the drawing.

The golf ball 2 has a diameter of 40 mm or greater but 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is preferably equal to or less than 44 mm and more preferably equal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g or greater but 50 g or less. In light of attainment of great inertia, the weight is preferably equal to or greater than 44 g and more preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is preferably equal to or less than 45.93 g.

FIG. 2 is a line graph showing a hardness distribution of the core 4 of the golf ball 2 in FIG. 1. The horizontal axis of the graph indicates the ratio (%) of a distance from the central point of the core 4 to the radius of the core 4. The vertical axis of the graph indicates a JIS-C hardness. Nine measuring points obtained by dividing a region from the central point of the core 4 to the surface of the core 4 at intervals of 12.5% of the radius of the core 4 are plotted in the graph. The ratio of the distance from the central point of the core 4 to each of these measuring points to the radius of the core 4 is as follows.

First point: 0.0% (central point)

Second point: 12.5%

Third point: 25.0%

Fourth point: 37.5%

Fifth point: 50.0%

Sixth point: 62.5%

Seventh point: 75.0%

Eighth point: 87.5%

Ninth point: 100.0% (surface)

Hardnesses at the first to eighth points are measured by pressing a JIS-C type hardness scale against a cut plane of the core 4 that has been cut into two halves. A hardness Hs at the ninth point is measured by pressing the JIS-C type hardness scale against the surface of the spherical core 4. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used. In the present invention, a JIS-C hardness at a measuring point whose distance from the central point of the core 4 is x (%) is represented by H(x). The hardness at the central point of the core 4 is represented by H(0).

FIG. 2 also shows a linear approximation curve obtained by a least-square method on the basis of the distances and the hardnesses of the nine measuring points. As is clear from FIG. 2, the broken line does not greatly deviate from the linear approximation curve. In other words, the broken line has a shape close to the linear approximation curve. In the core 4, the hardness linearly increases from its central point toward its surface. When the core 4 is hit with a middle iron, the energy loss is low. The core 4 has excellent resilience performance. When the golf ball 2 is hit with a middle iron, the flight distance is large.

In the core 4, R² of the linear approximation curve obtained by the least-square method is equal to or greater than 0.95. R² is an index indicating the linearity of the broken line. For the core 4 for which R² is equal to or greater than 0.95, the shape of the broken line of the hardness distribution is close to a straight line. The core 4 for which R² is equal to or greater than 0.95 has excellent resilience performance. R² is more preferably equal to or greater than 0.96 and particularly preferably equal to or greater than 0.97. R² is calculated by squaring a correlation coefficient R. The correlation coefficient R is calculated by dividing the covariance of the distance (%) from the central point and the hardness (JIS-C) by the standard deviation of the distance (%) from the central point and the standard deviation of the hardness (JIS-C).

The core 4 is obtained by crosslinking a rubber composition. The rubber composition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) an acid and/or a salt.

During heating and forming of the core 4, the base rubber (a) is crosslinked by the co-crosslinking agent (b). The heat of the crosslinking reaction remains near the central point of the core 4. Thus, during heating and forming of the core 4, the temperature at the central portion is high. The temperature gradually decreases from the central point toward the surface. It is inferred that in the rubber composition, the acid reacts with the metal salt of the co-crosslinking agent (b) to bond to cation. It is inferred that in the rubber composition, the salt reacts with the metal salt of the co-crosslinking agent (b) to exchange cation. By the bonding and the exchange, metal crosslinks are broken. This breaking is likely to occur in the central portion of the core 4 where the temperature is high, and is unlikely to occur near the surface of the core 4. As a result, the crosslinking density of the core 4 increases from its central point toward its surface. In the core 4, an outer-hard/inner-soft structure can be achieved. When the golf ball 2 that includes the core 4 is hit with a middle iron, the spin rate is low. The golf ball 2 achieves excellent flight performance upon a shot with a middle iron.

Examples of the base rubber (a) of the core 4 include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of resilience performance, polybutadienes are preferred. When a polybutadiene and another rubber are used in combination, it is preferred that the polybutadiene is included as a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and more preferably equal to or greater than 80% by weight. The proportion of cis-1,4 bonds in the polybutadiene is preferably equal to or greater than 40% by weight and more preferably equal to or greater than 80% by weight.

A polybutadiene in which the proportion of 1,2-vinyl bonds is equal to or less than 2.0% by weight is preferred. The polybutadiene can contribute to the resilience performance of the core 4. In this respect, the proportion of 1,2-vinyl bonds is preferably equal to or less than 1.7% by weight and particularly preferably equal to or less than 1.5% by weight.

From the standpoint that a polybutadiene having a low proportion of 1,2-vinyl bonds and excellent polymerization activity is obtained, a rare-earth-element-containing catalyst is preferably used for synthesis of a polybutadiene. In particular, a polybutadiene synthesized with a catalyst containing neodymium, which is a lanthanum-series rare earth element compound, is preferred.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of preferably 30 or greater, more preferably 32 or greater, and particularly preferably 35 or greater. The Mooney viscosity (ML₁₊₄ (100° C.)) is preferably equal to or less than 140, more preferably equal to or less than 120, even more preferably equal to or less than 100, and particularly preferably equal to or less than 80. The Mooney viscosity (ML₊₄ (100° C.)) is measured according to the standards of “JIS K6300”. The measurement conditions are as follows.

Rotor: L rotor

Preheating time: 1 minute

Rotating time of rotor: 4 minutes

Temperature: 100° C.

In light of workability, the polybutadiene has a molecular weight distribution (Mw/Mn) of preferably 2.0 or greater, more preferably 2.2 or greater, even more preferably 2.4 or greater, and particularly preferably 2.6 or greater. In light of resilience performance, the molecular weight distribution (Mw/Mn) is preferably equal to or less than 6.0, more preferably equal to or less than 5.0, even more preferably equal to or less than 4.0, and particularly preferably equal to or less than 3.4. The molecular weight distribution (Mw/Mn) is calculated by dividing the weight average molecular weight Mw by the number average molecular weight Mn.

The molecular weight distribution is measured by gel permeation chromatography (“HLC-8120GPC” manufactured by Tosoh Corporation). The measurement conditions are as follows.

Detector: differential refractometer

Column: GMHHXL (manufactured by Tosoh Corporation)

Column temperature: 40° C.

Mobile phase: tetrahydrofuran

The molecular weight distribution is calculated as a value obtained by conversion using polystyrene standard.

Preferable co-crosslinking agents (b) are:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

The rubber composition may include only the α,β-unsaturated carboxylic acid (b1) or only the metal salt (b2) of the α,β-unsaturated carboxylic acid as the co-crosslinking agent (b). The rubber composition may include both the α,β-unsaturated carboxylic acid (b1) and the metal salt (b2) of the α,β-unsaturated carboxylic acid as the co-crosslinking agent (b).

The metal salt (b2) of the α,β-unsaturated carboxylic acid graft-polymerizes with the molecular chain of the base rubber, thereby crosslinking the rubber molecules. When the rubber composition includes the α,β-unsaturated carboxylic acid (b1), the rubber composition preferably further includes a metal compound (f). The metal compound (f) reacts with the α,β-unsaturated carboxylic acid (b1) in the rubber composition. A salt obtained by this reaction graft-polymerizes with the molecular chain of the base rubber.

Examples of the metal compound (f) include metal hydroxides such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide; metal oxides such as magnesium oxide, calcium oxide, zinc oxide, and copper oxide; and metal carbonates such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, and potassium carbonate. A compound that includes a bivalent metal is preferred. The compound that includes the bivalent metal reacts with the co-crosslinking agent (b) to form metal crosslinks. The metal compound (f) is particularly preferably a zinc compound. Two or more metal compounds may be used in combination.

Examples of the α,β-unsaturated carboxylic acids include acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid. Examples of the metal component in the metal salt (b2) of the α,β-unsaturated carboxylic acid include sodium ion, potassium ion, lithium ion, magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, aluminum ion, tin ion, and zirconium ion. The metal salt (b2) of the α,β-unsaturated carboxylic acid may include two or more types of ions. From the standpoint that metal crosslinks are likely to occur between the rubber molecules, bivalent metal ions such as magnesium ion, calcium ion, zinc ion, barium ion, and cadmium ion are preferred. The metal salt (b2) of the α,β-unsaturated carboxylic acid is particularly preferably zinc acrylate.

In light of resilience performance of the golf ball 2, the amount of the co-crosslinking agent (b) is preferably equal to or greater than 15 parts by weight and particularly preferably equal to or greater than 20 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact, the amount is preferably equal to or less than 50 parts by weight, more preferably equal to or less than 45 parts by weight, and particularly preferably equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber.

The crosslinking initiator (c) is preferably an organic peroxide. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of preferable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of the crosslinking initiator (c) is preferably equal to or greater than 0.2 parts by weight and particularly preferably equal to or greater than 0.5 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact and durability of the golf ball 2, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.5 parts by weight, per 100 parts by weight of the base rubber.

The co-crosslinking agent (b) is not included in the concept of the acid and/or the salt (d). It is inferred that the acid and/or the salt (d) breaks the metal crosslinks by the co-crosslinking agent (b) in the central portion of the core 4 during heating and forming of the core 4. Examples of the acid and/or the salt (d) include oxo acids such as carboxylic acids, sulfonic acids, and phosphoric acid, and salts thereof; and hydroacids such as hydrochloric acid and hydrofluoric acid, and salts thereof. Oxo acids and salts thereof are preferred. A carboxylic acid and/or a salt thereof (dl) is more preferred. Carboxylates are particularly preferred.

The carbon number of the carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is preferably equal to or greater than 1 but equal to or less than 30, more preferably equal to or greater than 3 but equal to or less than 30, and even more preferably equal to or greater than 5 but equal to or less than 28. Examples of the carboxylic acid include aliphatic carboxylic acids (fatty acids) and aromatic carboxylic acids. Fatty acids and salts thereof are preferred.

The rubber composition may include a saturated fatty acid or a salt thereof, or may include an unsaturated fatty acid or a salt thereof. The saturated fatty acid and the salt thereof are preferred.

Examples of fatty acids include butyric acid (C4), valeric acid (C5), caproic acid (C6), enanthic acid (C7), caprylic acid (C8), pelargonic acid (C9), capric acid (C10), lauric acid (C12), myristic acid (C14), myristoleic acid (C14), pentadecylic acid (C15), palmitic acid (C16), palmitoleic acid (C16), margaric acid (C17), stearic acid (C18), elaidic acid (C18), vaccenic acid (C18), oleic acid (C18), linolic acid (C18), linolenic acid (C18), 12-hydroxystearic acid (C18), arachidic acid (C20), gadoleic acid (C20), arachidonic acid (C20), eicosenoic acid (C20), behenic acid (C22), erucic acid (C22), lignoceric acid (C24), nervonic acid (C24), cerotic acid (C26), montanic acid (C28), and melissic acid (C30). Two or more fatty acid salts may be used in combination. Caprylic acid (octanoic acid), lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, and behenic acid are preferred.

An aromatic carboxylic acid has an aromatic ring and a carboxyl group. Examples of aromatic carboxylic acids include benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, hemimellitic acid (benzene-1,2,3-tricarboxylic acid), trimellitic acid (benzene-1,2,4-tricarboxylic acid), trimesic acid (benzene-1,3,5-tricarboxylic acid), mellophanic acid (benzene-1,2,3,4-tetracarboxylic acid), prehnitic acid (benzene-1,2,3,5-tetracarboxylic acid), pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid), mellitic acid (benzene hexacarboxylic acid), diphenic acid (biphenyl-2,2′-dicarboxylic acid), toluic acid (methylbenzoic acid), xylic acid, prehnitylic acid (2,3,4-trimethylbenzoic acid), γ-isodurylic acid (2,3,5-trimethylbenzoic acid), durylic acid (2,4,5-trimethylbenzoic acid), β-isodurylic acid (2,4,6-trimethylbenzoic acid), α-isodurylic acid (3,4,5-trimethylbenzoic acid), cuminic acid (4-isopropylbenzoic acid), uvitic acid (5-methylisophthalic acid), α-toluic acid (phenylacetic acid), hydratropic acid (2-phenylpropanoic acid), and hydrocinnamic acid (3-phenylpropanoic acid).

The rubber composition may include an aromatic carboxylic acid substituted with a hydroxyl group, an alkoxy group, or an oxo group, or a salt thereof. Examples of this carboxylic acid can include salicylic acid (2-hydroxybenzoic acid), anisic acid (methoxybenzoic acid), cresotinic acid (hydroxy(methyl)benzoic acid), o-homosalicylic acid (2-hydroxy-3-methylbenzoic acid), m-homosalicylic acid (2-hydroxy-4-methylbenzoic acid), p-homosalicylic acid (2-hydroxy-5-methylbenzoic acid), o-pyrocatechuic acid (2,3-dihydroxybenzoic acid), β-resorcylic acid (2,4-dihydroxybenzoic acid), γ-resorcylic acid (2,6-dihydroxybenzoic acid), protocatechuic acid (3,4-dihydroxybenzoic acid), α-resorcylic acid (3,5-dihydroxybenzoic acid), vanillic acid (4-hydroxy-3-methoxybenzoic acid), isovanillic acid (3-hydroxy-4-methoxybenzoic acid), veratric acid (3,4-dimethoxybenzoic acid), o-veratric acid (2,3-dimethoxybenzoic acid), orsellinic acid (2,4-dihydroxy-6-methylbenzoic acid), m-hemipinic acid (4,5-dimethoxyphthalic acid), gallic acid (3,4,5-trihydroxybenzoic acid), syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid), asaronic acid (2,4,5-trimethoxybenzoic acid), mandelic acid (hydroxy(phenyl)acetic acid), vanillylmandelic acid (hydroxy(4-hydroxy-3-methoxyphenyl)acetic acid), homoanisic acid ((4-methoxyphenyl)acetic acid), homogentisic acid ((2,5-dihydroxyphenyl)acetic acid), homoprotocatechuic acid ((3,4-dihydroxyphenyl)acetic acid), homovanillic acid ((4-hydroxy-3-methoxyphenyl)acetic acid), homoisovanillic acid ((3-hydroxy-4-methoxyphenyl)acetic acid), homoveratric acid ((3,4-dimethoxyphenyl)acetic acid), o-homoveratric acid ((2,3-dimethoxyphenyl)acetic acid), homophthalic acid (2-(carboxymethyl)benzoic acid), homoisophthalic acid (3-(carboxymethyl)benzoic acid), homoterephthalic acid (4-(carboxymethyl)benzoic acid), phthalonic acid (2-(carboxycarbonyl)benzoic acid), isophthalonic acid (3-(carboxycarbonyl)benzoic acid), terephthalonic acid (4-(carboxycarbonyl)benzoic acid), benzilic acid (hydroxydiphenylacetic acid), atrolactic acid (2-hydroxy-2-phenylpropanoic acid), tropic acid (3-hydroxy-2-phenylpropanoic acid), melilotic acid (3-(2-hydroxyphenyl)propanoic acid), phloretic acid (3-(4-hydroxyphenyl)propanoic acid), hydrocaffeic acid (3-(3,4-dihydroxyphenyl)propanoic acid), hydroferulic acid (3-(4-hydroxy-3-methoxyphenyl)propanoic acid), hydroisoferulic acid (3-(3-hydroxy-4-methoxyphenyl)propanoic acid), p-coumaric acid (3-(4-hydroxyphenyl)acrylic acid), umbellic acid (3-(2,4-dihydroxyphenyl)acrylic acid), caffeic acid (3-(3,4-dihydroxyphenyl)acrylic acid), ferulic acid (3-(4-hydroxy-3-methoxyphenyl)acrylic acid), isoferulic acid (3-(3-hydroxy-4-methoxyphenyl)acrylic acid), and sinapic acid (3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic acid).

The cationic component of the carboxylate is a metal ion or an organic cation. Examples of the metal ion include sodium ion, potassium ion, lithium ion, silver ion, magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, copper ion, cobalt ion, nickel ion, manganese ion, aluminum ion, iron ion, tin ion, zirconium ion, and titanium ion. Two or more types of ions may be used in combination.

The organic cation has a carbon chain. Examples of the organic cation include organic ammonium ions. Examples of organic ammonium ions include primary ammonium ions such as stearylammonium ion, hexylammonium ion, octylammonium ion, and 2-ethylhexylammonium ion; secondary ammonium ions such as dodecyl(lauryl)ammonium ion, and octadecyl(stearyl)ammonium ion; tertiary ammonium ions such as trioctylammonium ion; and quaternary ammonium ions such as dioctyldimethylammonium ion, and distearyldimethylammonium ion. Two or more types of organic cations may be used in combination.

Examples of preferable carboxylates include a potassium salt, a magnesium salt, an aluminum salt, a zinc salt, an iron salt, a copper salt, a nickel salt, or a cobalt salt of octanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, or behenic acid. Zinc salts of carboxylic acids are particularly preferred. Specific examples of preferable carboxylates include zinc octoate, zinc laurate, zincmyristate, and zinc stearate.

In light of linearity of the hardness distribution of the core 4, the amount of the acid and/or the salt (d) is preferably equal to or greater than 0.5 parts by weight, more preferably equal to or greater than 1.0 parts by weight, and particularly preferably equal to or greater than 2.0 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably less than 40 parts by weight, more preferably equal to or less than 30 parts by weight, and particularly preferably equal to or less than 20 parts by weight, per 100 parts by weight of the base rubber.

The weight ratio of the co-crosslinking agent (b) and the acid and/or the salt (d) in the rubber composition is preferably equal to or greater than 3/7 but equal to or less than 9/1, and is particularly preferably equal to or greater than 4/6 but equal to or less than 8/2. From the rubber composition in which this weight ratio is within the above range, the core 4 whose hardness linearly increases from its central point toward its surface can be obtained.

As the co-crosslinking agent (b), zinc acrylate is preferably used. Zinc acrylate whose surface is coated with stearic acid or zinc stearate for the purpose of improving dispersibility to rubber is present. In the present invention, when the rubber composition includes this zinc acrylate, this coating material is not included in the concept of the acid and/or the salt (d).

The rubber composition preferably further includes an organic sulfur compound (e). The organic sulfur compound (e) can contribute to control of: the linearity of the hardness distribution of the core 4; and the degree of the outer-hard/inner-soft structure. An example of the organic sulfur compound (e) is an organic compound having a thiol group or a polysulfide linkage having 2 to 4 sulfur atoms. A metal salt of this organic compound is also included in the organic sulfur compound (e). Examples of the organic sulfur compound (e) include aliphatic compounds such as aliphatic thiols, aliphatic thiocarboxylic acids, aliphatic dithiocarboxylic acids, and aliphatic polysulfides; heterocyclic compounds; alicyclic compounds such as alicyclic thiols, alicyclic thiocarboxylic acids, alicyclic dithiocarboxylic acids, and alicyclic polysulfides; and aromatic compounds. Specific examples of the organic sulfur compound (e) include thiophenols, thionaphthols, polysulfides, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, thiurams, dithiocarbamates, and thiazoles. Preferable organic sulfur compounds (e) are thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.

Specific examples of the organic sulfur compound (e) are represented by the following chemical formulas (1) to (4).

In the chemical formula (1), R1 to R5 each represent H or a substituent.

In the chemical formula (2), R1 to R10 each represent H or a substituent.

In the chemical formula (3), R1 to R5 each represent H or a substituent, and M1 represents a monovalent metal atom.

In the chemical formula (4), R1 to R10 each represent H or a substituent, and M2 represents a bivalent metal atom.

In the formulas (1) to (4), each substituent is at least one group selected from the group consisting of a halogen group (F, Cl, Br, I), an alkyl group, a carboxyl group (—COOH), an ester (—COOR) of a carboxyl group, a formyl group (—CHO), an acyl group (—COR), a carbonyl halide group (—COX), a sulfo group (—SO₃H), an ester (—SO₃R) of a sulfo group, a sulfonyl halide group (—SO₂X), a sulfino group (—SO₂H), an alkylsulfinyl group (—SOR), a carbamoyl group (—CONH₂), an alkyl halide group, a cyano group (—CN), and an alkoxy group (—OR).

Examples of the organic sulfur compound represented by the chemical formula (1) include thiophenol; thiophenols substituted with halogen groups, such as 4-fluorothiophenol, 2,5-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, pentafluorothiophenol, 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, pentachlorothiophenol, 4-bromothiophenol, 2,5-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, pentabromothiophenol, 4-iodothiophenol, 2,5-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, and pentaiodothiophenol; thiophenols substituted with alkyl groups, such as 4-methylthiophenol, 2,4,5-trimethylthiophenol, pentamethylthiophenol, 4-t-butylthiophenol, 2,4,5-tri-t-butylthiophenol, and penta-t-butylthiophenol; thiophenols substituted with carboxyl groups, such as 4-carboxythiophenol, 2,4,6-tricarboxythiophenol, and pentacarboxythiophenol; thiophenols substituted with alkoxycarbonyl groups, such as 4-methoxycarbonylthiophenol, 2,4,6-trimethoxycarbonylthiophenol, and pentamethoxycarbonylthiophenol; thiophenols substituted with formyl groups, such as 4-formylthiophenol, 2,4,6-triformylthiophenol, and pentaformylthiophenol; thiophenols substituted with acyl groups, such as 4-acetylthiophenol, 2,4,6-triacetylthiophenol, and pentaacetylthiophenol; thiophenols substituted with carbonyl halide groups, such as 4-chlorocarbonylthiophenol, 2,4,6-tri(chlorocarbonyl)thiophenol, and penta(chlorocarbonyl)thiophenol; thiophenols substituted with sulfo groups, such as 4-sulfothiophenol, 2,4,6-trisulfothiophenol, and pentasulfothiophenol; thiophenols substituted with alkoxysulfonyl groups, such as 4-methoxysulfonylthiophenol, 2,4,6-trimethoxysulfonylthiophenol, and pentamethoxysulfonylthiophenol; thiophenols substituted with sulfonyl halide groups, such as 4-chlorosulfonylthiophenol, 2,4,6-tri(chlorosulfonyl)thiophenol, and penta(chlorosulfonyl)thiophenol; thiophenols substituted with sulfino groups, such as 4-sulfinothiophenol, 2,4,6-trisulfinothiophenol, and pentasulfinothiophenol; thiophenols substituted with alkylsulfinyl groups, such as 4-methylsulfinylthiophenol, 2,4,6-tri(methylsulfinyl)thiophenol, and penta(methylsulfinyl)thiophenol; thiophenols substituted with carbamoyl groups, such as 4-carbamoylthiophenol, 2,4,6-tricarbamoylthiophenol, and pentacarbamoylthiophenol; thiophenols substituted with alkyl halide groups, such as 4-trichloromethylthiophenol, 2,4,6-tri(trichloromethyl)thiophenol, and penta(trichloromethyl)thiophenol; thiophenols substituted with cyano groups, such as 4-cyanothiophenol, 2,4,6-tricyanothiophenol, and pentacyanothiophenol; and thiophenols substituted with alkoxy groups, such as 4-methoxythiophenol, 2,4,6-trimethoxythiophenol, and pentamethoxythiophenol. Each of these thiophenols is substituted with one type of substituent.

Another example of the organic sulfur compound represented by the chemical formula (1) is a compound substituted with at least one type of the above substituents and another substituent. Examples of the other substituent include a nitro group (—NO₂), an amino group (—NH₂), a hydroxyl group (—OH), and a phenylthio group (—SPh). Specific examples of the compound include 4-chloro-2-nitrothiophenol, 4-chloro-2-aminothiophenol, 4-chloro-2-hydroxythiophenol, 4-chloro-2-phenylthiothiophenol, 4-methyl-2-nitrothiophenol, 4-methyl-2-aminothiophenol, 4-methyl-2-hydroxythiophenol, 4-methyl-2-phenylthiothiophenol, 4-carboxy-2-nitrothiophenol, 4-carboxy-2-aminothiophenol, 4-carboxy-2-hydroxythiophenol, 4-carboxy-2-phenylthiothiophenol, 4-methoxycarbonyl-2-nitrothiophenol, 4-methoxycarbonyl-2-aminothiophenol, 4-methoxycarbonyl-2-hydroxythiophenol, 4-methoxycarbonyl-2-phenylthiothiophenol, 4-formyl-2-nitrothiophenol, 4-formyl-2-aminothiophenol, 4-formyl-2-hydroxythiophenol, 4-formyl-2-phenylthiothiophenol, 4-acetyl-2-nitrothiophenol, 4-acetyl-2-aminothiophenol, 4-acetyl-2-hydroxythiophenol, 4-acetyl-2-phenylthiothiophenol, 4-chlorocarbonyl-2-nitrothiophenol, 4-chlorocarbonyl-2-aminothiophenol, 4-chlorocarbonyl-2-hydroxythiophenol, 4-chlorocarbonyl-2-phenylthiothiophenol, 4-sulfo-2-nitrothiophenol, 4-sulfo-2-aminothiophenol, 4-sulfo-2-hydroxythiophenol, 4-sulfo-2-phenylthiothiophenol, 4-methoxysulfonyl-2-nitrothiophenol, 4-methoxysulfonyl-2-aminothiophenol, 4-methoxysulfonyl-2-hydroxythiophenol, 4-methoxysulfonyl-2-phenylthiothiophenol, 4-chlorosulfonyl-2-nitrothiophenol, 4-chlorosulfonyl-2-aminothiophenol, 4-chlorosulfonyl-2-hydroxythiophenol, 4-chlorosulfonyl-2-phenylthiothiophenol, 4-sulfino-2-nitrothiophenol, 4-sulfino-2-aminothiophenol, 4-sulfino-2-hydroxythiophenol, 4-sulfino-2-phenylthiothiophenol, 4-methylsulfinyl-2-nitrothiophenol, 4-methylsulfinyl-2-aminothiophenol, 4-methylsulfinyl-2-hydroxythiophenol, 4-methylsulfinyl-2-phenylthiothiophenol, 4-carbamoyl-2-nitrothiophenol, 4-carbamoyl-2-aminothiophenol, 4-carbamoyl-2-hydroxythiophenol, 4-carbamoyl-2-phenylthiothiophenol, 4-trichloromethyl-2-nitrothiophenol, 4-trichloromethyl-2-aminothiophenol, 4-trichloromethyl-2-hydroxythiophenol, 4-trichloromethyl-2-phenylthiothiophenol, 4-cyano-2-nitrothiophenol, 4-cyano-2-aminothiophenol, 4-cyano-2-hydroxythiophenol, 4-cyano-2-phenylthiothiophenol, 4-methoxy-2-nitrothiophenol, 4-methoxy-2-aminothiophenol, 4-methoxy-2-hydroxythiophenol, and 4-methoxy-2-phenylthiothiophenol.

Still another example of the organic sulfur compound represented by the chemical formula (1) is a compound substituted with two or more types of substituents. Specific examples of the compound include 4-acetyl-2-chlorothiophenol, 4-acetyl-2-methylthiophenol, 4-acetyl-2-carboxythiophenol, 4-acetyl-2-methoxycarbonylthiophenol, 4-acetyl-2-formylthiophenol, 4-acetyl-2-chlorocarbonylthiophenol, 4-acetyl-2-sulfothiophenol, 4-acetyl-2-methoxysulfonylthiophenol, 4-acetyl-2-chlorosulfonylthiophenol, 4-acetyl-2-sulfinothiophenol, 4-acetyl-2-methylsulfinylthiophenol, 4-acetyl-2-carbamoylthiophenol, 4-acetyl-2-trichloromethylthiophenol, 4-acetyl-2-cyanothiophenol, and 4-acetyl-2-methoxythiophenol.

Examples of the organic sulfur compound represented by the chemical formula (2) include diphenyl disulfide; diphenyl disulfides substituted with halogen groups, such as bis(4-fluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide, bis(2,4,5,6-tetrafluorophenyl)disulfide, bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(2,4,5-trichlorophenyl)disulfide, bis(2,4,5,6-tetrachlorophenyl)disulfide, bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(2,4,5-tribromophenyl)disulfide, bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide, bis(2,4,5-triiodophenyl)disulfide, bis(2,4,5,6-tetraiodophenyl)disulfide, and bis(pentaiodophenyl)disulfide; diphenyl disulfides substituted with alkyl groups, such as bis(4-methylphenyl)disulfide, bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide, bis(4-t-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide, and bis(penta-t-butylphenyl)disulfide; diphenyl disulfides substituted with carboxyl groups, such as bis(4-carboxyphenyl)disulfide, bis(2,4,6-tricarboxyphenyl)disulfide, and bis(pentacarboxyphenyl)disulfide; diphenyl disulfides substituted with alkoxycarbonyl groups, such as bis(4-methoxycarbonylphenyl)disulfide, bis(2,4,6-trimethoxycarbonylphenyl)disulfide, and bis(pentamethoxycarbonylphenyl)disulfide; diphenyl disulfides substituted with formyl groups, such as bis(4-formylphenyl)disulfide, bis(2,4,6-triformylphenyl)disulfide, and bis(pentaformylphenyl)disulfide; diphenyl disulfides substituted with acyl groups, such as bis(4-acetylphenyl)disulfide, bis(2,4,6-triacetylphenyl)disulfide, and bis(pentaacetylphenyl)disulfide; diphenyl disulfides substituted with carbonyl halide groups, such as bis(4-chlorocarbonylphenyl)disulfide, bis(2,4,6-tri(chlorocarbonyl)phenyl)disulfide, and bis(penta(chlorocarbonyl)phenyl)disulfide; diphenyl disulfides substituted with sulfo groups, such as bis(4-sulfophenyl)disulfide, bis(2,4,6-trisulfophenyl)disulfide, and bis(pentasulfophenyl)disulfide; diphenyl disulfides substituted with alkoxysulfonyl groups, such as bis(4-methoxysulfonylphenyl)disulfide, bis(2,4,6-trimethoxysulfonylphenyl)disulfide, and bis(pentamethoxysulfonylphenyl)disulfide; diphenyl disulfides substituted with sulfonyl halide groups, such as bis(4-chlorosulfonylphenyl)disulfide, bis(2,4,6-tri(chlorosulfonyl)phenyl)disulfide, and bis(penta(chlorosulfonyl)phenyl)disulfide; diphenyl disulfides substituted with sulfino groups, such as bis(4-sulfinophenyl)disulfide, bis(2,4,6-trisulfinophenyl)disulfide, and bis(pentasulfinophenyl)disulfide; diphenyl disulfides substituted with alkylsulfinyl groups, such as bis(4-methylsulfinylphenyl)disulfide, bis(2,4,6-tri(methylsulfinyl)phenyl)disulfide, and bis(penta(methylsulfinyl)phenyl)disulfide; diphenyl disulfides substituted with carbamoyl groups, such as bis(4-carbamoylphenyl)disulfide, bis(2,4,6-tricarbamoylphenyl)disulfide, and bis(pentacarbamoylphenyl)disulfide; diphenyl disulfides substituted with alkyl halide groups, such as bis(4-trichloromethylphenyl)disulfide, bis(2,4,6-tri(trichloromethyl)phenyl)disulfide, and bis(penta(trichloromethyl)phenyl)disulfide; diphenyl disulfides substituted with cyano groups, such as bis(4-cyanophenyl)disulfide, bis(2,4,6-tricyanophenyl)disulfide, and bis(pentacyanophenyl)disulfide; and diphenyl disulfides substituted with alkoxy groups, such as bis(4-methoxyphenyl)disulfide, bis(2,4,6-trimethoxyphenyl)disulfide, and bis(pentamethoxyphenyl)disulfide. Each of these diphenyl disulfides is substituted with one type of substituent.

Another example of the organic sulfur compound represented by the chemical formula (2) is a compound substituted with at least one type of the above substituents and another substituent. Examples of the other substituent include a nitro group (—NO₂), an amino group (—NH₂), a hydroxyl group (—OH), and a phenylthio group (—SPh). Specific examples of the compound include bis(4-chloro-2-nitrophenyl)disulfide, bis(4-chloro-2-aminophenyl)disulfide, bis(4-chloro-2-hydroxyphenyl)disulfide, bis(4-chloro-2-phenylthiophenyl)disulfide, bis(4-methyl-2-nitrophenyl)disulfide, bis(4-methyl-2-aminophenyl)disulfide, bis(4-methyl-2-hydroxyphenyl)disulfide, bis(4-methyl-2-phenylthiophenyl)disulfide, bis(4-carboxy-2-nitrophenyl)disulfide, bis(4-carboxy-2-aminophenyl)disulfide, bis(4-carboxy-2-hydroxyphenyl)disulfide, bis(4-carboxy-2-phenylthiophenyl)disulfide, bis(4-methoxycarbonyl-2-nitrophenyl)disulfide, bis(4-methoxycarbonyl-2-aminophenyl)disulfide, bis(4-methoxycarbonyl-2-hydroxyphenyl)disulfide, bis(4-methoxycarbonyl-2-phenylthiophenyl)disulfide, bis(4-formyl-2-nitrophenyl)disulfide, bis(4-formyl-2-aminophenyl)disulfide, bis(4-formyl-2-hydroxyphenyl)disulfide, bis(4-formyl-2-phenylthiophenyl)disulfide, bis(4-acetyl-2-nitrophenyl)disulfide, bis(4-acetyl-2-aminophenyl)disulfide, bis(4-acetyl-2-hydroxyphenyl)disulfide, bis(4-acetyl-2-phenylthiophenyl)disulfide, bis(4-chlorocarbonyl-2-nitrophenyl)disulfide, bis(4-chlorocarbonyl-2-aminophenyl)disulfide, bis(4-chlorocarbonyl-2-hydroxyphenyl)disulfide, bis(4-chlorocarbonyl-2-phenylthiophenyl)disulfide, bis(4-sulfo-2-nitrophenyl)disulfide, bis(4-sulfo-2-aminophenyl)disulfide, bis(4-sulfo-2-hydroxyphenyl)disulfide, bis(4-sulfo-2-phenylthiophenyl)disulfide, bis(4-methoxysulfonyl-2-nitrophenyl)disulfide, bis(4-methoxysulfonyl-2-aminophenyl)disulfide, bis(4-methoxysulfonyl-2-hydroxyphenyl)disulfide, bis(4-methoxysulfonyl-2-phenylthiophenyl)disulfide, bis(4-chlorosulfonyl-2-nitrophenyl)disulfide, bis(4-chlorosulfonyl-2-aminophenyl)disulfide, bis(4-chlorosulfonyl-2-hydroxyphenyl)disulfide, bis(4-chlorosulfonyl-2-phenylthiophenyl)disulfide, bis(4-sulfino-2-nitrophenyl)disulfide, bis(4-sulfino-2-aminophenyl)disulfide, bis(4-sulfino-2-hydroxyphenyl)disulfide, bis(4-sulfino-2-phenylthiophenyl)disulfide, bis(4-methylsulfinyl-2-nitrophenyl)disulfide, bis(4-methylsulfinyl-2-aminophenyl)disulfide, bis(4-methylsulfinyl-2-hydroxyphenyl)disulfide, bis(4-methylsulfinyl-2-phenylthiophenyl)disulfide, bis(4-carbamoyl-2-nitrophenyl)disulfide, bis(4-carbamoyl-2-aminophenyl)disulfide, bis(4-carbamoyl-2-hydroxyphenyl)disulfide, bis(4-carbamoyl-2-phenylthiophenyl)disulfide, bis(4-trichloromethyl-2-nitrophenyl)disulfide, bis(4-trichloromethyl-2-aminophenyl)disulfide, bis(4-trichloromethyl-2-hydroxyphenyl)disulfide, bis(4-trichloromethyl-2-phenylthiophenyl)disulfide, bis(4-cyano-2-nitrophenyl)disulfide, bis(4-cyano-2-aminophenyl)disulfide, bis(4-cyano-2-hydroxyphenyl)disulfide, bis(4-cyano-2-phenylthiophenyl)disulfide, bis(4-methoxy-2-nitrophenyl)disulfide, bis(4-methoxy-2-aminophenyl)disulfide, bis(4-methoxy-2-hydroxyphenyl)disulfide, and bis(4-methoxy-2-phenylthiophenyl)disulfide.

Still another example of the organic sulfur compound represented by the chemical formula (2) is a compound substituted with two or more types of substituents. Specific examples of the compound include bis(4-acetyl-2-chlorophenyl)disulfide, bis(4-acetyl-2-methylphenyl)disulfide, bis(4-acetyl-2-carboxyphenyl)disulfide, bis(4-acetyl-2-methoxycarbonylphenyl)disulfide, bis(4-acetyl-2-formylphenyl)disulfide, bis(4-acetyl-2-chlorocarbonylphenyl)disulfide, bis(4-acetyl-2-sulfophenyl)disulfide, bis(4-acetyl-2-methoxysulfonylphenyl)disulfide, bis(4-acetyl-2-chlorosulfonylphenyl)disulfide, bis(4-acetyl-2-sulfinophenyl)disulfide, bis(4-acetyl-2-methylsulfinylphenyl)disulfide, bis(4-acetyl-2-carbamoylphenyl)disulfide, bis(4-acetyl-2-trichloromethylphenyl)disulfide, bis(4-acetyl-2-cyanophenyl)disulfide, and bis(4-acetyl-2-methoxyphenyl)disulfide.

Examples of the organic sulfur compound represented by the chemical formula (3) include thiophenol sodium salt; thiophenol sodium salts substituted with halogen groups, such as 4-fluorothiophenol sodium salt, 2,5-difluorothiophenol sodium salt, 2,4,5-trifluorothiophenol sodium salt, 2,4,5,6-tetrafluorothiophenol sodium salt, pentafluorothiophenol sodium salt, 4-chlorothiophenol sodium salt, 2,5-dichlorothiophenol sodium salt, 2,4,5-trichlorothiophenol sodium salt, 2,4,5,6-tetrachlorothiophenol sodium salt, pentachlorothiophenol sodium salt, 4-bromothiophenol sodium salt, 2,5-dibromothiophenol sodium salt, 2,4,5-tribromothiophenol sodium salt, 2,4,5,6-tetrabromothiophenol sodium salt, pentabromothiophenol sodium salt, 4-iodothiophenol sodium salt, 2,5-diiodothiophenol sodium salt, 2,4,5-triiodothiophenol sodium salt, 2,4,5,6-tetraiodothiophenol sodium salt, and pentaiodothiophenol sodium salt; thiophenol sodium salts substituted with alkyl groups, such as 4-methylthiophenol sodium salt, 2,4,5-trimethylthiophenol sodium salt, pentamethylthiophenol sodium salt, 4-t-butylthiophenol sodium salt, 2,4,5-tri-t-butylthiophenol sodium salt, and penta(t-butyl)thiophenol sodium salt; thiophenol sodium salts substituted with carboxyl groups, such as 4-carboxythiophenol sodium salt, 2,4,6-tricarboxythiophenol sodium salt, and pentacarboxythiophenol sodium salt; thiophenol sodium salts substituted with alkoxycarbonyl groups, such as 4-methoxycarbonylthiophenol sodium salt, 2,4,6-trimethoxycarbonylthiophenol sodium salt, and pentamethoxycarbonylthiophenol sodium salt; thiophenol sodium salts substituted with formyl groups, such as 4-formylthiophenol sodium salt, 2,4,6-triformylthiophenol sodium salt, and pentaformylthiophenol sodium salt; thiophenol sodium salts substituted with acyl groups, such as 4-acetylthiophenolsodium salt, 2,4,6-triacetylthiophenol sodium salt, and pentaacetylthiophenol sodium salt; thiophenol sodium salts substituted with carbonyl halide groups, such as 4-chlorocarbonylthiophenol sodium salt, 2,4,6-tri(chlorocarbonyl)thiophenol sodium salt, and penta(chlorocarbonyl)thiophenol sodium salt; thiophenol sodium salts substituted with sulfo groups, such as 4-sulfothiophenol sodium salt, 2,4,6-trisulfothiophenol sodium salt, and pentasulfothiophenol sodium salt; thiophenol sodium salts substituted with alkoxysulfonyl groups, such as 4-methoxysulfonylthiophenol sodium salt, 2,4,6-trimethoxysulfonylthiophenol sodium salt, and pentamethoxysulfonylthiophenol sodium salt; thiophenol sodium salts substituted with sulfonyl halide groups, such as 4-chlorosulfonylthiophenol sodium salt, 2,4,6-tri(chlorosulfonyl)thiophenol sodium salt, and penta(chlorosulfonyl)thiophenol sodium salt; thiophenol sodium salts substituted with sulfino groups, such as 4-sulfinothiophenol sodium salt, 2,4,6-trisulfinothiophenol sodium salt, and pentasulfinothiophenol sodium salt; thiophenol sodium salts substituted with alkylsulfinyl groups, such as 4-methylsulfinylthiophenol sodium salt, 2,4,6-tri(methylsulfinyl)thiophenol sodium salt, and penta(methylsulfinyl)thiophenol sodium salt; thiophenol sodium salts substituted with carbamoyl groups, such as 4-carbamoylthiophenol sodium salt, 2,4,6-tricarbamoylthiophenol sodium salt, and pentacarbamoylthiophenol sodium salt; thiophenol sodium salts substituted with alkyl halide groups, such as 4-trichloromethylthiophenol sodium salt, 2,4,6-tri(trichloromethyl)thiophenol sodium salt, and penta(trichloromethyl)thiophenol sodium salt; thiophenol sodium salts substituted with cyano groups, such as 4-cyanothiophenol sodium salt, 2,4,6-tricyanothiophenol sodium salt, and pentacyanothiophenol sodium salt; and thiophenol sodium salts substituted with alkoxy groups, such as 4-methoxythiophenol sodium salt, 2,4,6-trimethoxythiophenol sodium salt, and pentamethoxythiophenol sodium salt. Each of these thiophenol sodium salts is substituted with one type of substituent.

Another example of the organic sulfur compound represented by the chemical formula (3) is a compound substituted with at least one type of the above substituents and another substituent. Examples of the other substituent include a nitro group (—NO₂), an amino group (—NH₂), a hydroxyl group (—OH), and a phenylthio group (—SPh). Specific examples of the compound include 4-chloro-2-nitrothiophenol sodium salt, 4-chloro-2-aminothiophenol sodium salt, 4-chloro-2-hydroxythiophenol sodium salt, 4-chloro-2-phenylthiothiophenol sodium salt, 4-methyl-2-nitrothiophenol sodium salt, 4-methyl-2-aminothiophenol sodium salt, 4-methyl-2-hydroxythiophenol sodium salt, 4-methyl-2-phenylthiothiophenol sodium salt, 4-carboxy-2-nitrothiophenol sodium salt, 4-carboxy-2-aminothiophenol sodium salt, 4-carboxy-2-hydroxythiophenol sodium salt, 4-carboxy-2-phenylthiothiophenol sodium salt, 4-methoxycarbonyl-2-nitrothiophenol sodium salt, 4-methoxycarbonyl-2-aminothiophenol sodium salt, 4-methoxycarbonyl-2-hydroxythiophenol sodium salt, 4-methoxycarbonyl-2-phenylthiothiophenol sodium salt, 4-formyl-2-nitrothiophenol sodium salt, 4-formyl-2-aminothiophenol sodium salt, 4-formyl-2-hydroxythiophenol sodium salt, 4-formyl-2-phenylthiothiophenol sodium salt, 4-acetyl-2-nitrothiophenol sodium salt, 4-acetyl-2-aminothiophenol sodium salt, 4-acetyl-2-hydroxythiophenol sodium salt, 4-acetyl-2-phenylthiothiophenol sodium salt, 4-chlorocarbonyl-2-nitrothiophenol sodium salt, 4-chlorocarbonyl-2-aminothiophenol sodium salt, 4-chlorocarbonyl-2-hydroxythiophenol sodium salt, 4-chlorocarbonyl-2-phenylthiothiophenol sodium salt, 4-sulfo-2-nitrothiophenol sodium salt, 4-sulfo-2-aminothiophenol sodium salt, 4-sulfo-2-hydroxythiophenol sodium salt, 4-sulfo-2-phenylthiothiophenol sodium salt, 4-methoxysulfonyl-2-nitrothiophenol sodium salt, 4-methoxysulfonyl-2-aminothiophenol sodium salt, 4-methoxysulfonyl-2-hydroxythiophenol sodium salt, 4-methoxysulfonyl-2-phenylthiothiophenol sodium salt, 4-chlorosulfonyl-2-nitrothiophenol sodium salt, 4-chlorosulfonyl-2-aminothiophenol sodium salt, 4-chlorosulfonyl-2-hydroxythiophenol sodium salt, 4-chlorosulfonyl-2-phenylthiothiophenol sodium salt, 4-sulfino-2-nitrothiophenol sodium salt, 4-sulfino-2-aminothiophenol sodium salt, 4-sulfino-2-hydroxythiophenol sodium salt, 4-sulfino-2-phenylthiothiophenol sodium salt, 4-methylsulfinyl-2-nitrothiophenol sodium salt, 4-methylsulfinyl-2-aminothiophenol sodium salt, 4-methylsulfinyl-2-hydroxythiophenol sodium salt, 4-methylsulfinyl-2-phenylthiothiophenol sodium salt, 4-carbamoyl-2-nitrothiophenol sodium salt, 4-carbamoyl-2-aminothiophenol sodium salt, 4-carbamoyl-2-hydroxythiophenol sodium salt, 4-carbamoyl-2-phenylthiothiophenol sodium salt, 4-trichloromethyl-2-nitrothiophenol sodium salt, 4-trichloromethyl-2-aminothiophenol sodium salt, 4-trichloromethyl-2-hydroxythiophenol sodium salt, 4-trichloromethyl-2-phenylthiothiophenol sodium salt, 4-cyano-2-nitrothiophenol sodium salt, 4-cyano-2-aminothiophenol sodium salt, 4-cyano-2-hydroxythiophenol sodium salt, 4-cyano-2-phenylthiothiophenol sodium salt, 4-methoxy-2-nitrothiophenol sodium salt, 4-methoxy-2-aminothiophenol sodium salt, 4-methoxy-2-hydroxythiophenol sodium salt, and 4-methoxy-2-phenylthiothiophenol sodium salt.

Still another example of the organic sulfur compound represented by the chemical formula (3) is a compound substituted with two or more types of substituents. Specific examples of the compound include 4-acetyl-2-chlorothiophenol sodium salt, 4-acetyl-2-methylthiophenol sodium salt, 4-acetyl-2-carboxythiophenol sodium salt, 4-acetyl-2-methoxycarbonylthiophenol sodium salt, 4-acetyl-2-formylthiophenol sodium salt, 4-acetyl-2-chlorocarbonylthiophenol sodium salt, 4-acetyl-2-sulfothiophenol sodium salt, 4-acetyl-2-methoxysulfonylthiophenol sodium salt, 4-acetyl-2-chlorosulfonylthiophenol sodium salt, 4-acetyl-2-sulfinothiophenol sodium salt, 4-acetyl-2-methylsulfinylthiophenol sodium salt, 4-acetyl-2-carbamoylthiophenol sodium salt, 4-acetyl-2-trichloromethylthiophenol sodium salt, 4-acetyl-2-cyanothiophenol sodium salt, and 4-acetyl-2-methoxythiophenol sodium salt. Examples of the monovalent metal represented by M1 in the chemical formula (3) include sodium, lithium, potassium, copper (I), and silver (I).

Examples of the organic sulfur compound represented by the chemical formula (4) include thiophenol zinc salt; thiophenol zinc salts substituted with halogen groups, such as 4-fluorothiophenol zinc salt, 2,5-difluorothiophenol zinc salt, 2,4,5-trifluorothiophenol zinc salt, 2,4,5,6-tetrafluorothiophenol zinc salt, pentafluorothiophenol zinc salt, 4-chlorothiophenol zinc salt, 2,5-dichlorothiophenol zinc salt, 2,4,5-trichlorothiophenol zinc salt, 2,4,5,6-tetrachlorothiophenol zinc salt, pentachlorothiophenol zinc salt, 4-bromothiophenol zinc salt, 2,5-dibromothiophenol zinc salt, 2,4,5-tribromothiophenol zinc salt, 2,4,5,6-tetrabromothiophenol zinc salt, pentabromothiophenol zinc salt, 4-iodothiophenol zinc salt, 2,5-diiodothiophenol zinc salt, 2,4,5-triiodothiophenol zinc salt, 2,4,5,6-tetraiodothiophenol zinc salt, and pentaiodothiophenol zinc salt; thiophenol zinc salts substituted with alkyl groups, such as 4-methylthiophenol zinc salt, 2,4,5-trimethylthiophenol zinc salt, pentamethylthiophenol zinc salt, 4-t-butylthiophenol zinc salt, 2,4,5-tri-t-butylthiophenol zinc salt, and penta-t-butylthiophenol zinc salt; thiophenol zinc salts substituted with carboxyl groups, such as 4-carboxythiophenol zinc salt, 2,4,6-tricarboxythiophenol zinc salt, and pentacarboxythiophenol zinc salt; thiophenol zinc salts substituted with alkoxycarbonyl groups, such as 4-methoxycarbonylthiophenol zinc salt, 2,4,6-trimethoxycarbonylthiophenol zinc salt, and pentamethoxycarbonylthiophenol zinc salt; thiophenol zinc salts substituted with formyl groups, such as 4-formylthiophenol zinc salt, 2,4,6-triformylthiophenol zinc salt, and pentaformylthiophenol zinc salt; thiophenol zinc salts substituted with acyl groups, such as 4-acetylthiophenol zinc salt, 2,4,6-triacetylthiophenol zinc salt, and pentaacetylthiophenol zinc salt; thiophenol zinc salts substituted with carbonyl halide groups, such as 4-chlorocarbonylthiophenol zinc salt, 2,4,6-tri(chlorocarbonyl)thiophenol zinc salt, and penta(chlorocarbonyl)thiophenol zinc salt; thiophenol zinc salts substituted with sulfo groups, such as 4-sulfothiophenol zinc salt, 2,4,6-trisulfothiophenol zinc salt, and pentasulfothiophenol zinc salt; thiophenol zinc salts substituted with alkoxysulfonyl groups, such as 4-methoxysulfonylthiophenol zinc salt, 2,4,6-trimethoxysulfonylthiophenol zinc salt, and pentamethoxysulfonylthiophenol zinc salt; thiophenol zinc salts substituted with sulfonyl halide groups, such as 4-chlorosulfonylthiophenol zinc salt, 2,4,6-tri(chlorosulfonyl)thiophenol zinc salt, and penta(chlorosulfonyl)thiophenol zinc salt; thiophenol zinc salts substituted with sulfino groups, such as 4-sulfinothiophenol zinc salt, 2,4,6-trisulfinothiophenol zinc salt, and pentasulfinothiophenol zinc salt; thiophenol zinc salts substituted with alkylsulfinyl groups, such as 4-methylsulfinylthiophenol zinc salt, 2,4,6-tri(methylsulfinyl)thiophenol zinc salt, and penta(methylsulfinyl)thiophenol zinc salt; thiophenol zinc salts substituted with carbamoyl groups, such as 4-carbamoylthiophenol zinc salt, 2,4,6-tricarbamoylthiophenol zinc salt, and pentacarbamoylthiophenol zinc salt; thiophenol zinc salts substituted with alkyl halide groups, such as 4-trichloromethylthiophenol zinc salt, 2,4,6-tri(trichloromethyl)thiophenol zinc salt, and penta(trichloromethyl)thiophenol zinc salt; thiophenol zinc salts substituted with cyano groups, such as 4-cyanothiophenol zinc salt, 2,4,6-tricyanothiophenol zinc salt, and pentacyanothiophenol zinc salt; and thiophenol zinc salts substituted with alkoxy groups, such as 4-methoxythiophenol zinc salt, 2,4,6-trimethoxythiophenol zinc salt, and pentamethoxythiophenol zinc salt. Each of these thiophenol zinc salts is substituted with one type of substituent.

Another example of the organic sulfur compound represented by the chemical formula (4) is a compound substituted with at least one type of the above substituents and another substituent. Examples of the other substituent include a nitro group (—NO₂), an amino group (—NH₂), a hydroxyl group (—OH), and a phenylthio group (—SPh). Specific examples of the compound include 4-chloro-2-nitrothiophenol zinc salt, 4-chloro-2-aminothiophenol zinc salt, 4-chloro-2-hydroxythiophenol zinc salt, 4-chloro-2-phenylthiothiophenol zinc salt, 4-methyl-2-nitrothiophenol zinc salt, 4-methyl-2-aminothiophenol zinc salt, 4-methyl-2-hydroxythiophenol zinc salt, 4-methyl-2-phenylthiothiophenol zinc salt, 4-carboxy-2-nitrothiophenol zinc salt, 4-carboxy-2-aminothiophenol zinc salt, 4-carboxy-2-hydroxythiophenol zinc salt, 4-carboxy-2-phenylthiothiophenol zinc salt, 4-methoxycarbonyl-2-nitrothiophenol zinc salt, 4-methoxycarbonyl-2-aminothiophenol zinc salt, 4-methoxycarbonyl-2-hydroxythiophenol zinc salt, 4-methoxycarbonyl-2-phenylthiothiophenol zinc salt, 4-formyl-2-nitrothiophenol zinc salt, 4-formyl-2-aminothiophenol zinc salt, 4-formyl-2-hydroxythiophenol zinc salt, 4-formyl-2-phenylthiothiophenol zinc salt, 4-acetyl-2-nitrothiophenol zinc salt, 4-acetyl-2-aminothiophenol zinc salt, 4-acetyl-2-hydroxythiophenol zinc salt, 4-acetyl-2-phenylthiothiophenol zinc salt, 4-chlorocarbonyl-2-nitrothiophenol zinc salt, 4-chlorocarbonyl-2-aminothiophenol zinc salt, 4-chlorocarbonyl-2-hydroxythiophenol zinc salt, 4-chlorocarbonyl-2-phenylthiothiophenol zinc salt, 4-sulfo-2-nitrothiophenol zinc salt, 4-sulfo-2-aminothiophenol zinc salt, 4-sulfo-2-hydroxythiophenol zinc salt, 4-sulfo-2-phenylthiothiophenol zinc salt, 4-methoxysulfonyl-2-nitrothiophenol zinc salt, 4-methoxysulfonyl-2-aminothiophenol zinc salt, 4-methoxysulfonyl-2-hydroxythiophenol zinc salt, 4-methoxysulfonyl-2-phenylthiothiophenol zinc salt, 4-chlorosulfonyl-2-nitrothiophenol zinc salt, 4-chlorosulfonyl-2-aminothiophenol zinc salt, 4-chlorosulfonyl-2-hydroxythiophenol zinc salt, 4-chlorosulfonyl-2-phenylthiothiophenol zinc salt, 4-sulfino-2-nitrothiophenol zinc salt, 4-sulfino-2-aminothiophenol zinc salt, 4-sulfino-2-hydroxythiophenol zinc salt, 4-sulfino-2-phenylthiothiophenol zinc salt, 4-methylsulfinyl-2-nitrothiophenol zinc salt, 4-methylsulfinyl-2-aminothiophenol zinc salt, 4-methylsulfinyl-2-hydroxythiophenol zinc salt, 4-methylsulfinyl-2-phenylthiothiophenol zinc salt, 4-carbamoyl-2-nitrothiophenol zinc salt, 4-carbamoyl-2-aminothiophenol zinc salt, 4-carbamoyl-2-hydroxythiophenol zinc salt, 4-carbamoyl-2-phenylthiothiophenol zinc salt, 4-trichloromethyl-2-nitrothiophenol zinc salt, 4-trichloromethyl-2-aminothiophenol zinc salt, 4-trichloromethyl-2-hydroxythiophenol zinc salt, 4-trichloromethyl-2-phenylthiothiophenol zinc salt, 4-cyano-2-nitrothiophenol zinc salt, 4-cyano-2-aminothiophenol zinc salt, 4-cyano-2-hydroxythiophenol zinc salt, 4-cyano-2-phenylthiothiophenol zinc salt, 4-methoxy-2-nitrothiophenol zinc salt, 4-methoxy-2-aminothiophenol zinc salt, 4-methoxy-2-hydroxythiophenol zinc salt, and 4-methoxy-2-phenylthiothiophenol zinc salt.

Still another example of the organic sulfur compound represented by the chemical formula (4) is a compound substituted with two or more types of substituents. Specific examples of the compound include 4-acetyl-2-chlorothiophenol zinc salt, 4-acetyl-2-methylthiophenol zinc salt, 4-acetyl-2-carboxythiophenol zinc salt, 4-acetyl-2-methoxycarbonylthiophenol zinc salt, 4-acetyl-2-formylthiophenol zinc salt, 4-acetyl-2-chlorocarbonylthiophenol zinc salt, 4-acetyl-2-sulfothiophenol zinc salt, 4-acetyl-2-methoxysulfonylthiophenol zinc salt, 4-acetyl-2-chlorosulfonylthiophenol zinc salt, 4-acetyl-2-sulfinothiophenol zinc salt, 4-acetyl-2-methylsulfinylthiophenol zinc salt, 4-acetyl-2-carbamoylthiophenol zinc salt, 4-acetyl-2-trichloromethylthiophenol zinc salt, 4-acetyl-2-cyanothiophenol zinc salt, and 4-acetyl-2-methoxythiophenol zinc salt. Examples of the bivalent metal represented by M2 in the chemical formula (4) include zinc, magnesium, calcium, strontium, barium, titanium (II), manganese (II), iron (II), cobalt (II), nickel (II), zirconium (II), and tin (II).

Examples of naphthalenethiols include 2-naphthalenethiol, 1-naphthalenethiol, 2-chloro-1-naphthalenethiol, 2-bromo-1-naphthalenethiol, 2-fluoro-1-naphthalenethiol, 2-cyano-1-naphthalenethiol, 2-acetyl-1-naphthalenethiol, 1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol, 1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, 1-acetyl-2-naphthalenethiol, and metal salts thereof. 1-naphthalenethiol, 2-naphthalenethiol, and zinc salts thereof are preferred.

Examples of sulfenamide type organic sulfur compounds include N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and N-t-butyl-2-benzothiazole sulfenamide. Examples of thiuram type organic sulfur compounds include tetramethylthiuram monosulfide, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide. Examples of dithiocarbamates include zinc dimethyldithiocarbamate, zinc diethyldithiocarbamate, zinc dibutyldithiocarbamate, zinc ethylphenyldithiocarbamate, sodium dimethyldithiocarbamate, sodium diethyldithiocarbamate, copper (II) dimethyldithiocarbamate, iron (III) dimethyldithiocarbamate, selenium diethyldithiocarbamate, and tellurium diethyldithiocarbamate. Examples of thiazole type organic sulfur compounds include 2-mercaptobenzothiazole (MBT); dibenzothiazyl disulfide (MBTS); a sodium salt, a zinc salt, a copper salt, or a cyclohexylamine salt of 2-mercaptobenzothiazole; 2-(2,4-dinitrophenyl)mercaptobenzothiazole; and 2-(2,6-diethyl-4-morpholinothio)benzothiazole.

In light of resilience performance, the amount of the organic sulfur compound (e) is preferably equal to or greater than 0.05 parts by weight and particularly preferably equal to or greater than 0.1 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.0 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler may be included in the core 4. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 4 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, sulfur, a vulcanization accelerator, and the like are added to the rubber composition of the core 4. Crosslinked rubber powder or synthetic resin powder may also be dispersed in the rubber composition.

In the core 4, the difference (Hs−H(0)) between the surface hardness Hs and the central hardness H(0) is preferably equal to or greater than 15. The difference is great. In other words, the core 4 has an outer-hard/inner-soft structure. When the core 4 is hit with a middle iron, the recoil (torsional return) is great, and thus spin is suppressed. The core 4 contributes to the flight performance of the golf ball 2. In light of flight performance, the difference (Hs−H(0)) is more preferably equal to or greater than 20 and particularly preferably equal to or greater than 25. From the standpoint that the core 4 can easily be formed, the difference (Hs−H(0)) is preferably equal to or less than 50.

The hardness H(0) at the central point of the core 4 is preferably equal to or greater than 40.0 but equal to or less than 70.0. The golf ball 2 having a hardness H(0) of 40.0 or greater has excellent resilience performance. In this respect, the hardness H(0) is more preferably equal to or greater than 45.0 and particularly preferably equal to or greater than 50.0. The core 4 having a hardness H(0) of 70.0 or less can achieve an outer-hard/inner-soft structure. In the golf ball 2 that includes the core 4, spin can be suppressed. In this respect, the hardness H(0) is more preferably equal to or less than 68.0 and particularly preferably equal to or less than 66.0.

The hardness Hs at the surface of the core 4 is preferably equal to or greater than 76.0 but equal to or less than 95.0. In the core 4 having a hardness Hs of 76.0 or greater, an outer-hard/inner-soft structure can be achieved. In the golf ball 2 that includes the core 4, spin can be suppressed. In this respect, the hardness Hs is more preferably equal to or greater than 78.0 and particularly preferably equal to or greater than 80.0. The golf ball 2 having a hardness Hs of 95.0 or less has excellent durability. In this respect, the hardness Hs is more preferably equal to or less than 93.0 and particularly preferably equal to or less than 90.0.

The core 4 preferably has a diameter of 36.0 mm or greater but 41.0 mm or less. The core 4 having a diameter of 36.0 mm or greater can achieve excellent resilience performance of the golf ball 2. In this respect, the diameter is more preferably equal to or greater than 36.5 mm and particularly preferably equal to or greater than 37.0 mm. In the golf ball 2 that includes the core 4 having a diameter of 41.0 mm or less, the inner cover 8, the mid cover 10, and the outer cover 12 can have sufficient thicknesses. The golf ball 2 that includes the inner cover 8, the mid cover 10, and the outer cover 12 which have large thicknesses has excellent durability. In this respect, the diameter is more preferably equal to or less than 40.0 mm and particularly preferably equal to or less than 39.0 mm. The core 4 may have a rib on the surface thereof.

In light of feel at impact, the core 4 has an amount of compressive deformation (comp'n) of preferably 3.0 mm or greater and particularly preferably 3.3 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 4.6 mm and particularly preferably equal to or less than 4.3 mm.

For the inner cover 8, a resin composition is suitably used. Examples of the base polymer of the resin composition include ionomer resins, styrene block-containing thermoplastic elastomers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, and thermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. The golf ball 2 that includes the inner cover 8 including an ionomer resin has excellent resilience performance. An ionomer resin and another resin may be used in combination for the inner cover 8. In this case, the principal component of the base polymer is preferably the ionomer resin. Specifically, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or greater but 90% by weight or less of an α-olefin, and 10% by weight or greater but 20% by weight or less of an α,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or greater but 85% by weight or less of an α-olefin, 5% by weight or greater but 30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% by weight or greater but 25% by weight or less of an α,β-unsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymers and the ternary copolymers, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferable ionomer resin is a copolymer formed with ethylene and acrylic acid or methacrylic acid.

In the binary copolymers and the ternary copolymers, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions in light of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7318”, “Himilan AM7329”, “Himilan AM7337”, “Himilan MK7320”, and “Himilan MK7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK7010”, “IOTEK7030”, “IOTEK7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Company.

Two or more ionomer resins may be used in combination for the inner cover 8. An ionomer resin neutralized with a monovalent metal ion, and an ionomer resin neutralized with a bivalent metal ion may be used in combination.

A preferable resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer has excellent compatibility with ionomer resins. A resin composition including the styrene block-containing thermoplastic elastomer has excellent fluidity.

The styrene block-containing thermoplastic elastomer includes a polystyrene block as a hard segment, and a soft segment. A typical soft segment is a diene block. Examples of compounds for the diene block include butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferred. Two or more compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers include styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenated SBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenated SBS include styrene-ethylene-butylene-styrene block copolymers (SEBS). Examples of hydrogenated SIS include styrene-ethylene-propylene-styrene block copolymers (SEPS). Examples of hydrogenated SIBS include styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content of the styrene component in the styrene block-containing thermoplastic elastomer is preferably equal to or greater than 10% by weight, more preferably equal to or greater than 12% by weight, and particularly preferably equal to or greater than 15% by weight. In light of feel at impact of the golf ball 2, the content is preferably equal to or less than 50% by weight, more preferably equal to or less than 47% by weight, and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplastic elastomers include an alloy of an olefin and one or more members selected from the group consisting of SBS, SIS, SIBS, SEGS, SEPS, and SEEPS. The olefin component in the alloy is presumed to contribute to improvement of compatibility with ionomer resins. Use of this alloy improves the resilience performance of the golf ball 2. An olefin having 2 to 10 carbon atoms is preferably used. Examples of suitable olefins include ethylene, propylene, butene, and pentene. Ethylene and propylene are particularly preferred.

Specific examples of polymer alloys include trade names “Rabalon T3221C”, “Rabalon T3339C”, “Rabalon SJ4400N”, “Rabalon SJ5400N”, “Rabalon SJ6400N”, “Rabalon SJ7400N”, “Rabalon SJ8400N”, “Rabalon SJ9400N”, and “Rabalon SR04”, manufactured by Mitsubishi Chemical Corporation. Other specific examples of styrene block-containing thermoplastic elastomers include trade name “Epofriend A1010” manufactured by Daicel Chemical Industries, Ltd., and trade name “Septon HG-252” manufactured by Kuraray Co., Ltd.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the resin composition of the inner cover 8 in an adequate amount. The inner cover 8 may include powder of a metal with a high specific gravity.

According to the finding by the inventor of the present invention, the inner cover 8 greatly influences the deformation behavior of the golf ball 2 when the golf ball 2 is hit with a middle iron. The golf ball 2 that includes the inner cover 8 having a great hardness Hi has excellent flight performance when being hit with a middle iron. The inner cover 8 having a hardness Hi that is not excessively great can contribute to the feel at impact of the golf ball 2.

In light of flight performance, the hardness Hi of the inner cover 8 is preferably equal to or greater than 75, more preferably equal to or greater than 80, and particularly preferably equal to or greater than 83. In light of feel at impact of the golf ball 2, the hardness Hi is preferably equal to or less than 95 and particularly preferably equal to or less than 90. The hardness Hi is measured with a JIS-C type hardness scale mounted to an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.). For the measurement, a slab that is formed by hot press and that has a thickness of about 2 mm is used. A slab kept at 23° C. for two weeks is used for the measurement. At the measurement, three slabs are stacked. A slab formed from the same resin composition as the resin composition of the inner cover 8 is used.

From the standpoint that an outer-hard/inner-soft structure is achieved in the sphere consisting of the core 4 and the inner cover 8 and spin of the golf ball 2 is suppressed, the hardness Hi of the inner cover 8 is preferably greater than the surface hardness Hs of the core 4. In light of suppression of spin, the difference (Hi−Hs) between the hardness Hi and the hardness Hs is preferably equal to or greater than 1 and particularly preferably equal to or greater than 2. The difference (Hi−Hs) is preferably equal to or less than 10. In the sphere in which the difference (Hi−Hs) is equal to or less than 10, the hardness linearly increases from its central point toward its surface. In the sphere whose hardness linearly increases, the energy loss is low when the golf ball 2 is hit with a middle iron.

The inner cover 8 preferably has a thickness of 0.2 mm or greater but 1.2 mm or less. The inner cover 8 having a thickness of 0.2 mm or greater can easily be formed. In this respect, the thickness is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 2 that includes the inner cover 8 having a thickness of 1.2 mm or less has excellent resilience performance. In this respect, the thickness is more preferably equal to or less than 1.0 mm and particularly preferably equal to or less than 0.9 mm.

For forming the inner cover 8, known methods such as injection molding, compression molding, and the like can be used.

For the mid cover 10, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 2 that includes the mid cover 10 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the inner cover 8 can be used for the mid cover 10.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable other resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the inner cover 8 can be used for the mid cover 10.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the mid cover 10 in an adequate amount.

The mid cover 10 preferably has a JIS-C hardness Hm of 75 or greater but 96 or less. The golf ball 2 that includes the mid cover 10 having a hardness Hm of 75 or greater has excellent flight performance upon a shot with a middle iron. In this respect, the hardness Hm is more preferably equal to or greater than 80 and particularly preferably equal to or greater than 85. The golf ball 2 that includes the mid cover 10 having a hardness Hm of 96 or less has excellent feel at impact. In this respect, the hardness Hm is more preferably equal to or less than 90 and particularly preferably equal to or less than 88. The hardness Hm is measured by the same measurement method as that for the hardness Hi.

The hardness Hm of the mid cover 10 may be greater than the hardness Hi of the inner cover 8 or may be less than the hardness Hi of the inner cover 8.

In the golf ball 2 in which the hardness Hm is greater than the hardness Hi, an outer-hard/inner-soft structure of the sphere consisting of the core 4, the inner cover 8, and the mid cover 10 can be achieved. When the golf ball 2 is hit with a middle iron, the spin rate is low. The golf ball 2 has excellent flight performance upon a shot with a middle iron. In light of flight performance, the difference (Hm−Hi) between the hardness Hm and the hardness Hi is preferably equal to or greater than 2. The difference (Hm−Hi) is preferably equal to or less than 20.

In the golf ball 2 in which the hardness Hm is less than the hardness Hi, the mid cover 10 absorbs the shock when the golf ball 2 is hit. The golf ball 2 has excellent feel at impact. In light of feel at impact, the difference (Hi−Hm) between the hardness Hi and the hardness Hm is preferably equal to or greater than 2. The difference (Hi−Hm) is preferably equal to or less than 20.

The mid cover 10 preferably has a thickness of 0.2 mm or greater but 1.2 mm or less. The mid cover 10 having a thickness of 0.2 mm or greater can easily be formed. In this respect, the thickness is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 2 that includes the mid cover 10 having a thickness of 1.2 mm or less has excellent resilience performance. In this respect, the thickness is more preferably equal to or less than 1.0 mm and particularly preferably equal to or less than 0.9 mm.

For forming the mid cover 10, known methods such as injection molding, compression molding, and the like can be used.

For the outer cover 12, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 2 that includes the outer cover 12 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the inner cover 8 can be used for the outer cover 12.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable resin that can be used in combination with an ionomer resin is an ethylene-(meth)acrylic acid copolymer. The copolymer is obtained by a copolymerization reaction of a monomer composition that contains ethylene and (meth)acrylic acid. In the copolymer, some of the carboxyl groups are neutralized with metal ions. The copolymer includes 3% by weight or greater but 25% by weight or less of a (meth)acrylic acid component. An ethylene-(meth)acrylic acid copolymer having a polar functional group is particularly preferred. A specific example of ethylene-(meth)acrylic acid copolymers is trade name “NUCREL” manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.

Another preferable resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the inner cover 8 can be used for the outer cover 12.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the outer cover 12 in an adequate amount.

The outer cover 12 preferably has a JIS-C hardness Ho of 83 or greater but 98 or less. The golf ball 2 that includes the outer cover 12 having a hardness Ho of 83 or greater has excellent resilience performance. The golf ball 2 has excellent flight performance. In this respect, the hardness Ho is more preferably equal to or greater than 86 and particularly preferably equal to or greater than 89. The golf ball 2 that includes the outer cover 12 having a hardness Ho of 98 or less has excellent feel at impact. In this respect, the hardness Ho is more preferably equal to or less than 96 and particularly preferably equal to or less than 94. The hardness Ho is measured by the same measurement method as that for the hardness Hi.

The hardness Ho of the outer cover 12 is preferably greater than the hardness Hi of the inner cover 8. In the golf ball 2 in which the hardness Ho is greater than the hardness Hi, spin can be suppressed. The golf ball 2 has excellent flight performance upon a shot with a middle iron.

In light of flight performance, the difference (Ho−Hi) between the hardness Ho and the hardness Hi is preferably equal to or greater than 5, more preferably equal to or greater than 6, and particularly preferably equal to or greater than 7. The difference (Ho−Hi) is preferably equal to or less than 30.

The hardness Ho of the outer cover 12 is preferably greater than the hardness Hm of the mid cover 10. In the golf ball 2 in which the hardness Ho is greater than the hardness Hm, an outer-hard/inner-soft structure can be achieved. When the golf ball 2 is hit with a middle iron, the spin rate is low. The golf ball 2 has excellent flight performance upon a shot with a middle iron. In light of flight performance, the difference (Ho−Hm) between the hardness Ho and the hardness Hm is preferably equal to or greater than 2. The difference (Ho−Hm) is preferably equal to or less than 20.

The outer cover 12 preferably has a thickness of 0.2 mm or greater but 1.2 mm or less. The outer cover 12 having a thickness of 0.2 mm or greater can easily be formed. In this respect, the thickness is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 2 that includes the outer cover 12 having a thickness of 1.2 mm or less has excellent resilience performance. In this respect, the thickness is more preferably equal to or less than 1.0 mm and particularly preferably equal to or less than 0.9 mm.

For forming the outer cover 12, known methods such as injection molding, compression molding, and the like can be used. When forming the outer cover 12, the dimples 14 are formed by pimples formed on the cavity face of a mold.

The cover 6 has a total thickness of preferably 2.5 mm or less. The golf ball 2 that includes the cover 6 having a total thickness of 2.5 mm or less has excellent feel at impact. In this respect, the total thickness is particularly preferably equal to or less than 2.4 mm. In light of durability of the golf ball 2, the total thickness is preferably equal to or greater than 0.3 mm, more preferably equal to or greater than 0.5 mm, and particularly preferably equal to or greater than 0.8 mm.

In light of feel at impact, the golf ball 2 has an amount of compressive deformation (comp'n) of preferably 2.8 mm or greater, more preferably 2.9 mm or greater, and particularly preferably 3.0 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 3.6 mm, more preferably equal to or less than 3.5 mm, and particularly preferably equal to or less than 3.4 mm.

For measurement of the amount of compressive deformation (comp'n), a YAMADA type compression tester is used. In the tester, a sphere such as the core 4, the golf ball 2, or the like is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the sphere. The sphere, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the sphere up to the state in which a final load of 1274 N is applied thereto, is measured.

Second Embodiment

FIG. 3 shows a golf ball 102 according to a second embodiment of the present invention. The golf ball 102 includes a spherical core 104 and a cover 106 covering the core 104. The cover 106 includes a first cover 108, a second cover 110 positioned outside the first cover 108, a third cover 112 positioned outside the second cover 110, and a fourth cover 114 positioned outside the third cover 112. On the surface of the fourth cover 114, a large number of dimples 116 are formed. Of the surface of the golf ball 102, a part other than the dimples 116 is a land 118. The golf ball 102 includes a paint layer and a mark layer on the external side of the fourth cover 114, but these layers are not shown in the drawing.

The golf ball 102 has a diameter of 40 mm or greater but 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is preferably equal to or less than 44 mm and more preferably equal to or less than 42.80 mm. The golf ball 102 has a weight of 40 g or greater but 50 g or less. In light of attainment of great inertia, the weight is preferably equal to or greater than 44 g and more preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is preferably equal to or less than 45.93 g.

FIG. 4 is a line graph showing a hardness distribution of the core 104 of the golf ball 102 in FIG. 3. The horizontal axis of the graph indicates the ratio (%) of a distance from the central point of the core 104 to the radius of the core 104. The vertical axis of the graph indicates a JIS-C hardness. Nine measuring points obtained by dividing a region from the central point of the core 104 to the surface of the core 104 at intervals of 12.5% of the radius of the core 104 are plotted in the graph. The ratio of the distance from the central point of the core 104 to each of these measuring points to the radius of the core 104 is as follows.

First point: 0.0% (central point)

Second point: 12.5%

Third point: 25.0%

Fourth point: 37.5%

Fifth point: 50.0%

Sixth point: 62.5%

Seventh point: 75.0%

Eighth point: 87.5%

Ninth point: 100.0% (surface)

Hardnesses at the first to eighth points are measured by pressing a JIS-C type hardness scale against a cut plane of the core 104 that has been cut into two halves. A hardness Hs at the ninth point is measured by pressing the JIS-C type hardness scale against the surface of the spherical core 104. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used. In the present invention, a JIS-C hardness at a measuring point whose distance from the central point of the core 104 is x (%) is represented by H(x). The hardness at the central point of the core 104 is represented by H(0).

A straight line indicated by a dotted line in FIG. 4 is a linear approximation curve obtained by a least-square method on the basis of the distances and the hardnesses of the nine measuring points. As is clear from FIG. 4, the broken line does not greatly deviate from the linear approximation curve. In other words, the broken line has a shape close to the linear approximation curve. In the core 104, the hardness linearly increases from its central point toward its surface. When the core 104 is hit with a middle iron, the energy loss is low. The core 104 has excellent resilience performance. When the golf ball 102 is hit with a middle iron, the flight distance is large.

In the core 104, R² of the linear approximation curve obtained by the least-square method is equal to or greater than 0.95. R² is an index indicating the linearity of the broken line. For the core 104 for which R² is equal to or greater than 0.95, the shape of the broken line of the hardness distribution is close to a straight line. The core 104 for which R² is equal to or greater than 0.95 has excellent resilience performance. R² is more preferably equal to or greater than 0.96 and particularly preferably equal to or greater than 0.97. R² is calculated by squaring a correlation coefficient R. The correlation coefficient R is calculated by dividing the covariance of the distance (mm) from the central point and the hardness (JIS-C) by the standard deviation of the distance (mm) from the central point and the standard deviation of the hardness (JIS-C).

The core 104 is obtained by crosslinking a rubber composition. The rubber composition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) an acid and/or a salt.

During heating and forming of the core 104, the base rubber (a) is crosslinked by the co-crosslinking agent (b). The heat of the crosslinking reaction remains near the central point of the core 104. Thus, during heating and forming of the core 104, the temperature at the central portion is high. The temperature gradually decreases from the central point toward the surface. It is inferred that in the rubber composition, the acid reacts with the metal salt of the co-crosslinking agent (b) to bond to cation. It is inferred that in the rubber composition, the salt reacts with the metal salt of the co-crosslinking agent (b) to exchange cation. By the bonding and the exchange, metal crosslinks are broken. This breaking is likely to occur in the central portion of the core 104 where the temperature is high, and is unlikely to occur near the surface of the core 104. As a result, the crosslinking density of the core 104 increases from its central point toward its surface. In the core 104, an outer-hard/inner-soft structure can be achieved. When the golf ball 102 that includes the core 104 is hit with a middle iron, the spin rate is low. The golf ball 102 achieves excellent flight performance upon a shot with a middle iron.

The rubber composition of the core 104 can include the base rubber (a) described above for the core 4 of the first embodiment.

The rubber composition of the core 104 can include the co-crosslinking agent (b) described above for the core 4 of the first embodiment. Preferable co-crosslinking agents (b) are:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

When the rubber composition includes the α,β-unsaturated carboxylic acid (b1), the rubber composition preferably further includes a metal compound (f). The metal compound (f) reacts with the α,β-unsaturated carboxylic acid (b1) in the rubber composition. A salt obtained by this reaction graft-polymerizes with the molecular chain of the base rubber. The rubber composition of the core 104 can include the metal compound (f) described above for the core 4 of the first embodiment.

In light of resilience performance of the golf ball 102, the amount of the co-crosslinking agent (b) is preferably equal to or greater than 15 parts by weight and particularly preferably equal to or greater than 20 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact, the amount is preferably equal to or less than 50 parts by weight, more preferably equal to or less than 45 parts by weight, and particularly preferably equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber.

The rubber composition of the core 104 can include the crosslinking initiator (c) described above for the core 4 of the first embodiment. In light of resilience performance of the golf ball 102, the amount of the crosslinking initiator (c) is preferably equal to or greater than 0.2 parts by weight and particularly preferably equal to or greater than 0.5 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact and durability of the golf ball 102, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.5 parts by weight, per 100 parts by weight of the base rubber.

The rubber composition of the core 104 can include the acid and/or the salt (d) described above for the core 4 of the first embodiment. The co-crosslinking agent (b) is not included in the concept of the acid and/or the salt (d). It is inferred that the acid and/or the salt (d) breaks the metal crosslinks by the co-crosslinking agent (b) in the central portion of the core 104 during heating and forming of the core 104.

In light of linearity of the hardness distribution of the core 104, the amount of the acid and/or the salt (d) is preferably equal to or greater than 1.0 parts by weight, more preferably equal to or greater than 2.0 parts by weight, and particularly preferably equal to or greater than 3.0 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 40 parts by weight, more preferably equal to or less than 35 parts by weight, and particularly preferably equal to or less than 25 parts by weight, per 100 parts by weight of the base rubber.

The weight ratio of the co-crosslinking agent (b) and the acid and/or the salt (d) in the rubber composition is preferably equal to or greater than 3/7 but equal to or less than 9/1, and is particularly preferably equal to or greater than 4/6 but equal to or less than 8/2. From the rubber composition in which this weight ratio is within the above range, the core 104 whose hardness linearly increases from its central point toward its surface can be obtained.

As the co-crosslinking agent (b), zinc acrylate is preferably used. Zinc acrylate whose surface is coated with stearic acid or zinc stearate for the purpose of improving dispersibility to rubber is present. In the present invention, when the rubber composition includes this zinc acrylate, this coating material is not included in the concept of the acid and/or the salt (d).

The rubber composition of the core 104 can include the organic sulfur compound (e) described above for the core 4 of the first embodiment. The organic sulfur compound (e) can contribute to control of: the linearity of the hardness distribution of the core 104; and the degree of the outer-hard/inner-soft structure. In light of resilience performance, the amount of the organic sulfur compound (e) is preferably equal to or greater than 0.05 parts by weight and particularly preferably equal to or greater than 0.1 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.0 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler may be included in the core 104. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 104 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, sulfur, a vulcanization accelerator, and the like are added to the rubber composition of the core 104. Crosslinked rubber powder or synthetic resin powder may also be dispersed in the rubber composition.

In the core 104, the difference (Hs−H(0)) between the surface hardness Hs and the central hardness H(0) is preferably equal to or greater than 15. The difference is great. In other words, the core 104 has an outer-hard/inner-soft structure. When the core 104 is hit with a middle iron, the recoil (torsional return) is great, and thus spin is suppressed. The core 104 contributes to the flight performance of the golf ball 102. In light of flight performance, the difference (Hs−H(0)) is more preferably equal to or greater than 20 and particularly preferably equal to or greater than 25. From the standpoint that the core 104 can easily be formed, the difference (Hs−H(0)) is preferably equal to or less than 50.

The hardness H(0) at the central point of the core 104 is preferably equal to or greater than 40.0 but equal to or less than 70.0. The golf ball 102 having a hardness H(0) of 40.0 or greater has excellent resilience performance. In this respect, the hardness H(0) is more preferably equal to or greater than 45.0 and particularly preferably equal to or greater than 50.0. In the core 104 having a hardness H(0) of 70.0 or less, an outer-hard/inner-soft structure can be achieved. In the golf ball 102 that includes the core 104, spin can be suppressed. In this respect, the hardness H(0) is more preferably equal to or less than 68.0 and particularly preferably equal to or less than 66.0.

The hardness Hs at the surface of the core 104 is preferably equal to or greater than 76.0 but equal to or less than 95.0. In the core 104 having a hardness Hs of 76.0 or greater, an outer-hard/inner-soft structure can be achieved. In the golf ball 102 that includes the core 104, spin can be suppressed. In this respect, the hardness Hs is more preferably equal to or greater than 78.0 and particularly preferably equal to or greater than 80.0. The golf ball 102 having a hardness Hs of 95.0 or less has excellent durability. In this respect, the hardness Hs is more preferably equal to or less than 93.0 and particularly preferably equal to or less than 90.0.

The core 104 preferably has a diameter of 35.0 mm or greater but 40.0 mm or less. The core 104 having a diameter of 35.0 mm or greater can achieve excellent resilience performance of the golf ball 102. In this respect, the diameter is more preferably equal to or greater than 35.5 mm and particularly preferably equal to or greater than 36.0 mm. In the golf ball 102 that includes the core 104 having a diameter of 40.0 mm or less, the first cover 108, the second cover 110, the third cover 112, and the fourth cover 114 can have sufficient thicknesses. The golf ball 102 that includes these covers 108, 110, 112, and 114 having large thicknesses has excellent durability. In this respect, the diameter is more preferably equal to or less than 39.0 mm and particularly preferably equal to or less than 38.0 mm. The core 104 may have a rib on the surface thereof.

In light of feel at impact, the core 104 has an amount of compressive deformation (comp'n) of preferably 3.0 mm or greater and particularly preferably 3.3 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 4.6 mm and particularly preferably equal to or less than 4.3 mm.

For the first cover 108, a resin composition is suitably used. Examples of the base polymer of the resin composition include ionomer resins, styrene block-containing thermoplastic elastomers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, and thermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. The golf ball 102 that includes the first cover 108 including an ionomer resin has excellent resilience performance. An ionomer resin and another resin may be used in combination for the first cover 108. In this case, the principal component of the base polymer is preferably the ionomer resin. Specifically, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

The first cover 108 can include the ionomer resin described above for the golf ball 2 of the first embodiment. The first cover 108 can include the styrene block-containing thermoplastic elastomer described above for the golf ball 2 of the first embodiment.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the resin composition of the first cover 108 in an adequate amount. The first cover 108 may include powder of a metal with a high specific gravity.

According to the finding by the inventor of the present invention, the first cover 108 greatly influences the deformation behavior of the golf ball 102 when the golf ball 102 is hit with a middle iron. The golf ball 102 that includes the first cover 108 having a great hardness H1 has excellent flight performance when being hit with a middle iron. The first cover 108 having a hardness H1 that is not excessively great can contribute to the feel at impact of the golf ball 102.

In light of flight performance, the hardness H1 of the first cover 108 is preferably equal to or greater than 75, more preferably equal to or greater than 80, and particularly preferably equal to or greater than 83. In light of feel at impact of the golf ball 102, the hardness H1 is preferably equal to or less than 95 and particularly preferably equal to or less than 90. The hardness H1 is measured with a JIS-C type hardness scale mounted to an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.). For the measurement, a slab that is formed by hot press and that has a thickness of about 2 mm is used. A slab kept at 23° C. for two weeks is used for the measurement. At the measurement, three slabs are stacked. A slab formed from the same resin composition as the resin composition of the first cover 108 is used.

From the standpoint that an outer-hard/inner-soft structure is achieved in the sphere consisting of the core 104 and the first cover 108 and spin of the golf ball 102 is suppressed, the hardness H1 of the first cover 108 is preferably greater than the surface hardness Hs of the core 104. In light of suppression of spin, the difference (H1−Hs) between the hardness H1 and the hardness Hs is preferably equal to or greater than 1 and particularly preferably equal to or greater than 2. The difference (H1−Hs) is preferably equal to or less than 10. In the sphere in which the difference (H1−Hs) is equal to or less than 10, the hardness linearly increases from its central point toward its surface. In the sphere whose hardness linearly increases, the energy loss is low when the golf ball 102 is hit with a middle iron.

The first cover 108 preferably has a thickness T1 of 0.2 mm or greater but 1.8 mm or less. The first cover 108 having a thickness T1 of 0.2 mm or greater can easily be formed. In this respect, the thickness T1 is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 102 that includes the first cover 108 having a thickness T1 of 1.8 mm or less has excellent resilience performance. In this respect, the thickness T1 is more preferably equal to or less than 1.4 mm and particularly preferably equal to or less than 1.2 mm.

For forming the first cover 108, known methods such as injection molding, compression molding, and the like can be used.

For the second cover 110, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 102 that includes the second cover 110 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the first cover 108 can be used for the second cover 110.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable other resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the first cover 108 can be used for the second cover 110.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the second cover 110 in an adequate amount.

The second cover 110 preferably has a JIS-C hardness H2 of 75 or greater but 96 or less. The golf ball 102 that includes the second cover 110 having a hardness H2 of 75 or greater has excellent flight performance upon a shot with a middle iron. In this respect, the hardness H2 is more preferably equal to or greater than 80 and particularly preferably equal to or greater than 85. The golf ball 102 that includes the second cover 110 having a hardness H2 of 96 or less has excellent feel at impact. In this respect, the hardness H2 is more preferably equal to or less than 94 and particularly preferably equal to or less than 92. The hardness H2 is measured by the same measurement method as that for the hardness H1.

The hardness H2 of the second cover 110 is preferably greater than the hardness H1 of the first cover 108. In the golf ball 102 in which the hardness H2 is greater than the hardness H1, an outer-hard/inner-soft structure of the sphere consisting of the core 104, the first cover 108, and the second cover 110 can be achieved. When the golf ball 102 is hit with a middle iron, the spin rate is low. The golf ball 102 has excellent flight performance upon a shot with a middle iron. In light of flight performance, the difference (H2−H1) between the hardness H2 and the hardness H1 is preferably equal to or greater than 2. The difference (H2−H1) is preferably equal to or less than 20.

The hardness H2 of the second cover 110 may be less than the hardness H1 of the first cover 108. The second cover 110 absorbs the shock when the golf ball 102 is hit. The golf ball 102 has excellent feel at impact. In light of feel at impact, the difference (H1−H2) between the hardness H1 and the hardness H2 is preferably equal to or greater than 2. The difference (H1−H2) is preferably equal to or less than 20.

The second cover 110 preferably has a thickness T2 of 0.2 mm or greater but 1.8 mm or less. The second cover 110 having a thickness T2 of 0.2 mm or greater can easily be formed. In this respect, the thickness T2 is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 102 that includes the second cover 110 having a thickness T2 of 1.8 mm or less has excellent resilience performance. In this respect, the thickness T2 is more preferably equal to or less than 1.4 mm and particularly preferably equal to or less than 1.2 mm.

For forming the second cover 110, known methods such as injection molding, compression molding, and the like can be used.

For the third cover 112, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 102 that includes the third cover 112 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the first cover 108 can be used for the third cover 112.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable other resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the first cover 108 can be used for the third cover 112.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the third cover 112 in an adequate amount.

The third cover 112 preferably has a JIS-C hardness H3 of 75 or greater but 96 or less. The golf ball 102 that includes the third cover 112 having a hardness H3 of 75 or greater has excellent flight performance upon a shot with a middle iron. In this respect, the hardness H3 is more preferably equal to or greater than 80 and particularly preferably equal to or greater than 85. The golf ball 102 that includes the third cover 112 having a hardness H3 of 96 or less has excellent feel at impact. In this respect, the hardness H3 is more preferably equal to or less than 94 and particularly preferably equal to or less than 92. The hardness H3 is measured by the same measurement method as that for the hardness H1.

The hardness H3 of the third cover 112 is preferably greater than the hardness H2 of the second cover 110. In the golf ball 102 in which the hardness H3 is greater than the hardness H2, an outer-hard/inner-soft structure of the sphere consisting of the core 104, the first cover 108, the second cover 110, and the third cover 112 can be achieved. When the golf ball 102 is hit with a middle iron, the spin rate is low. The golf ball 102 has excellent flight performance upon a shot with a middle iron. In light of flight performance, the difference (H3−H2) between the hardness H3 and the hardness H2 is preferably equal to or greater than 2. The difference (H3−H2) is preferably equal to or less than 20.

The hardness H3 of the third cover 112 may be less than the hardness H2 of the first cover 108. The third cover 112 absorbs the shock when the golf ball 102 is hit. The golf ball 102 has excellent feel at impact. In light of feel at impact, the difference (H2−H3) between the hardness H2 and the hardness H3 is preferably equal to or greater than 2. The difference (H2−H3) is preferably equal to or less than 20.

The third cover 112 preferably has a thickness T3 of 0.2 mm or greater but 1.8 mm or less. The third cover 112 having a thickness T3 of 0.2 mm or greater can easily be formed. In this respect, the thickness T3 is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 102 that includes the third cover 112 having a thickness T3 of 1.8 mm or less has excellent resilience performance. In this respect, the thickness T3 is more preferably equal to or less than 1.4 mm and particularly preferably equal to or less than 1.2 mm.

For forming the third cover 112, known methods such as injection molding, compression molding, and the like can be used.

For the fourth cover 114, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 102 that includes the fourth cover 114 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the first cover 108 can be used for the fourth cover 114.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable resin that can be used in combination with an ionomer resin is an ethylene-(meth)acrylic acid copolymer. The copolymer is obtained by a copolymerization reaction of a monomer composition that contains ethylene and (meth)acrylic acid. In the copolymer, some of the carboxyl groups are neutralized with metal ions. The copolymer includes 3% by weight or greater but 25% by weight or less of a (meth)acrylic acid component. An ethylene-(meth)acrylic acid copolymer having a polar functional group is particularly preferred. A specific example of ethylene-(meth)acrylic acid copolymers is trade name “NUCREL” manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.

Another preferable resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the first cover 108 can be used for the fourth cover 114.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the fourth cover 114 in an adequate amount.

The fourth cover 114 preferably has a JIS-C hardness H4 of 83 or greater but 98 or less. The golf ball 102 that includes the fourth cover 114 having a hardness H4 of 83 or greater has excellent resilience performance. The golf ball 102 has excellent flight performance. In this respect, the hardness H4 is more preferably equal to or greater than 86 and particularly preferably equal to or greater than 89. The golf ball 102 that includes the fourth cover 114 having a hardness H4 of 98 or less has excellent feel at impact. In this respect, the hardness H4 is more preferably equal to or less than 96 and particularly preferably equal to or less than 94. The hardness H4 is measured by the same measurement method as that for the hardness H1.

The hardness H4 of the fourth cover 114 is preferably greater than the hardness H1 of the first cover 108. In the golf ball 102 in which the hardness H4 is greater than the hardness H1, spin can be suppressed. The golf ball 102 has excellent flight performance upon a shot with a middle iron.

In light of flight performance, the difference (H4−H1) between the hardness H4 and the hardness H1 is preferably equal to or greater than 3, more preferably equal to or greater than 5, and particularly preferably equal to or greater than 6. The difference (H4−H1) is preferably equal to or less than 30.

The hardness H4 of the fourth cover 114 is preferably greater than the hardness H3 of the third cover 112. In the golf ball 102 in which the hardness H4 is greater than the hardness H3, an outer-hard/inner-soft structure can be achieved. When the golf ball 102 is hit with a middle iron, the spin rate is low. The golf ball 102 has excellent flight performance upon a shot with a middle iron. In light of flight performance, the difference (H4−H3) between the hardness H4 and the hardness H3 is preferably equal to or greater than 2. The difference (H4−H3) is preferably equal to or less than 20.

The fourth cover 114 preferably has a thickness T4 of 0.2 mm or greater but 1.8 mm or less. The fourth cover 114 having a thickness T4 of 0.2 mm or greater can easily be formed. In this respect, the thickness T4 is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 102 that includes the fourth cover 114 having a thickness T4 of 1.8 mm or less has excellent resilience performance. In this respect, the thickness T4 is more preferably equal to or less than 1.4 mm and particularly preferably equal to or less than 1.2 mm.

For forming the fourth cover 114, known methods such as injection molding, compression molding, and the like can be used. When forming the fourth cover 114, the dimples 116 are formed by pimples formed on the cavity face of a mold.

The cover 106 has a total thickness of preferably 6.0 mm or less. The golf ball 102 that includes the cover 106 having a total thickness of 6.0 mm or less has excellent feel at impact. In this respect, the total thickness is particularly preferably equal to or less than 4.0 mm. In light of durability of the golf ball 102, the total thickness is preferably equal to or greater than 0.4 mm, more preferably equal to or greater than 0.8 mm, and particularly preferably equal to or greater than 1.2 mm.

In light of feel at impact, the golf ball 102 has an amount of compressive deformation (comp'n) of preferably 2.8 mm or greater, more preferably 2.9 mm or greater, and particularly preferably 3.0 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 3.6 mm, more preferably equal to or less than 3.5 mm, and particularly preferably equal to or less than 3.4 mm.

Third Embodiment

FIG. 5 shows a golf ball 202 according to a third embodiment of the present invention. The golf ball 202 includes a spherical core 204 and a cover 206 covering the core 204. The cover 206 includes an inner cover 208, a mid cover 210 positioned outside the inner cover 208, and an outer cover 212 positioned outside the mid cover 210. On the surface of the outer cover 212, a large number of dimples 214 are formed. Of the surface of the golf ball 202, a part other than the dimples 214 is a land 216. The golf ball 202 includes a paint layer and a mark layer on the external side of the outer cover 212, but these layers are not shown in the drawing.

The golf ball 202 has a diameter of 40 mm or greater but 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is preferably equal to or less than 44 mm and more preferably equal to or less than 42.80 mm. The golf ball 202 has a weight of 40 g or greater but 50 g or less. In light of attainment of great inertia, the weight is preferably equal to or greater than 44 g and more preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is preferably equal to or less than 45.93 g.

FIG. 6 is a line graph showing a hardness distribution of the core 204 of the golf ball 202 in FIG. 5. The horizontal axis of the graph indicates the ratio (%) of a distance from the central point of the core 204 to the radius of the core 204. The vertical axis of the graph indicates a JIS-C hardness. Nine measuring points obtained by dividing a region from the central point of the core 204 to the surface of the core 204 at intervals of 12.5% of the radius of the core 204 are plotted in the graph. The ratio of the distance from the central point of the core 204 to each of these measuring points to the radius of the core 204 is as follows.

First point: 0.0% (central point)

Second point: 12.5%

Third point: 25.0%

Fourth point: 37.5%

Fifth point: 50.0%

Sixth point: 62.5%

Seventh point: 75.0%

Eighth point: 87.5%

Ninth point: 100.0% (surface)

Hardnesses at the first to eighth points are measured by pressing a JIS-C type hardness scale against a cut plane of the core 204 that has been cut into two halves. A hardness at the ninth point is measured by pressing the JIS-C type hardness scale against the surface of the spherical core 204. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used. In the present invention, a JIS-C hardness at a measuring point whose distance from the central point of the core 204 is x (%) is represented by H (x). The hardness at the central point of the core 204 is represented by H(0). It is noted that the hardness at the surface of the core 204 is represented by Hs.

FIG. 6 also shows a linear approximation curve obtained by a least-square method on the basis of the distances and the hardnesses of the nine measuring points. As is clear from FIG. 6, the broken line does not greatly deviate from the linear approximation curve. In other words, the broken line has a shape close to the linear approximation curve. In the core 204, the hardness linearly increases from its central point toward its surface. When the core 204 is hit with a middle iron, the energy loss is low. The core 204 has excellent resilience performance. When the golf ball 202 is hit with a middle iron, the flight distance is large.

In the core 204, R² of the linear approximation curve obtained by the least-square method is equal to or greater than 0.95. R² is an index indicating the linearity of the broken line. For the core 204 for which R² is equal to or greater than 0.95, the shape of the broken line of the hardness distribution is close to a straight line. The core 204 for which R² is equal to or greater than 0.95 has excellent resilience performance. R² is more preferably equal to or greater than 0.96 and particularly preferably equal to or greater than 0.97. R² is calculated by squaring a correlation coefficient R. The correlation coefficient R is calculated by dividing the covariance of the distance (%) from the central point and the hardness (JIS-C) by the standard deviation of the distance (%) from the central point and the standard deviation of the hardness (JIS-C).

The core 204 is obtained by crosslinking a rubber composition. The rubber composition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) an acid and/or a salt.

During heating and forming of the core 204, the base rubber (a) is crosslinked by the co-crosslinking agent (b). The heat of the crosslinking reaction remains near the central point of the core 204. Thus, during heating and forming of the core 204, the temperature at the central portion is high. The temperature gradually decreases from the central point toward the surface. It is inferred that in the rubber composition, the acid reacts with the metal salt of the co-crosslinking agent (b) to bond to cation. It is inferred that in the rubber composition, the salt reacts with the metal salt of the co-crosslinking agent (b) to exchange cation. By the bonding and the exchange, metal crosslinks are broken. This breaking is likely to occur in the central portion of the core 204 where the temperature is high, and is unlikely to occur near the surface of the core 204. As a result, the crosslinking density of the core 204 increases from its central point toward its surface. In the core 204, an outer-hard/inner-soft structure can be achieved. When the golf ball 202 that includes the core 204 is hit with a middle iron, the spin rate is low. The golf ball 202 achieves excellent flight performance upon a shot with a middle iron.

The rubber composition of the core 204 can include the base rubber (a) described above for the core 4 of the first embodiment.

The rubber composition of the core 204 can include the co-crosslinking agent (b) described above for the core 4 of the first embodiment. Preferable co-crosslinking agents (b) are:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

When the rubber composition includes the α,β-unsaturated carboxylic acid (31), the rubber composition preferably further includes a metal compound (f). The metal compound (f) reacts with the α,β-unsaturated carboxylic acid (b1) in the rubber composition. A salt obtained by this reaction graft-polymerizes with the molecular chain of the base rubber. The rubber composition of the core 204 can include the metal compound (f) described above for the core 4 of the first embodiment.

In light of resilience performance of the golf ball 202, the amount of the co-crosslinking agent (b) is preferably equal to or greater than 15 parts by weight and particularly preferably equal to or greater than 20 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact, the amount is preferably equal to or less than 50 parts by weight, more preferably equal to or less than 45 parts by weight, and particularly preferably equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber.

The rubber composition of the core 204 can include the crosslinking initiator (c) described above for the core 4 of the first embodiment. In light of resilience performance of the golf ball 202, the amount of the crosslinking initiator (c) is preferably equal to or greater than 0.2 parts by weight and particularly preferably equal to or greater than 0.5 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact and durability of the golf ball 202, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.5 parts by weight, per 100 parts by weight of the base rubber.

The rubber composition of the core 204 can include the acid and/or the salt (d) described above for the core 4 of the first embodiment. The co-crosslinking agent (b) is not included in the concept of the acid and/or the salt (d). It is inferred that the acid and/or the salt (d) breaks the metal crosslinks by the co-crosslinking agent (b) in the central portion of the core 204 during heating and forming of the core 204.

In light of linearity of the hardness distribution of the core 204, the amount of the acid and/or the salt (d) is preferably equal to or greater than 0.5 parts by weight, more preferably equal to or greater than 1.0 parts by weight, and even more preferably equal to or greater than 2.0 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 40 parts by weight and more preferably equal to or less than 30 parts by weight, per 100 parts by weight of the base rubber.

The weight ratio of the co-crosslinking agent (b) and the acid and/or the salt (d) in the rubber composition is preferably equal to or greater than 3/7 but equal to or less than 9/1, and is particularly preferably equal to or greater than 4/6 but equal to or less than 8/2. From the rubber composition in which this weight ratio is within the above range, the core 204 whose hardness linearly increases from its central point toward its surface can be obtained.

As the co-crosslinking agent (b), zinc acrylate is preferably used. Zinc acrylate whose surface is coated with stearic acid or zinc stearate for the purpose of improving dispersibility to rubber is present. In the present invention, when the rubber composition includes this zinc acrylate, this coating material is not included in the concept of the acid and/or the salt (d).

The rubber composition of the core 204 can include the organic sulfur compound (e) described above for the core 4 of the first embodiment. The organic sulfur compound (e) can contribute to control of: the linearity of the hardness distribution of the core 204; and the degree of the outer-hard/inner-soft structure. In light of resilience performance, the amount of the organic sulfur compound (e) is preferably equal to or greater than 0.05 parts by weight and particularly preferably equal to or greater than 0.1 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.0 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler may be included in the core 204. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 204 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, sulfur, a vulcanization accelerator, and the like are added to the rubber composition of the core 204. Crosslinked rubber powder or synthetic resin powder may also be dispersed in the rubber composition.

In the core 204, the difference (Hs−H(0)) between the surface hardness Hs and the central hardness H(0) is preferably equal to or greater than 15. The difference is great. In other words, the core 204 has an outer-hard/inner-soft structure. When the core 204 is hit with a middle iron, the recoil (torsional return) is great, and thus spin is suppressed. The core 204 contributes to the flight performance of the golf ball 202. In light of flight performance, the difference (Hs−H(0)) is more preferably equal to or greater than 20 and particularly preferably equal to or greater than 25. From the standpoint that the core 204 can easily be formed, the difference (Hs−H(0)) is preferably equal to or less than 50.

The hardness H(0) at the central point of the core 204 is preferably equal to or greater than 40.0 but equal to or less than 70.0. The golf ball 202 having a hardness H(0) of 40.0 or greater has excellent resilience performance. In this respect, the hardness H(0) is more preferably equal to or greater than 45.0 and particularly preferably equal to or greater than 50.0. The core 204 having a hardness H(0) of 70.0 or less can achieve an outer-hard/inner-soft structure. In the golf ball 202 that includes the core 204, spin can be suppressed. In this respect, the hardness H(0) is more preferably equal to or less than 68.0 and particularly preferably equal to or less than 66.0.

The hardness Hs at the surface of the core 204 is preferably equal to or greater than 76.0 but equal to or less than 95.0. In the core 204 having a hardness Hs of 76.0 or greater, an outer-hard/inner-soft structure can be achieved. In the golf ball 202 that includes the core 204, spin can be suppressed. In this respect, the hardness Hs is more preferably equal to or greater than 78.0 and particularly preferably equal to or greater than 80.0. The golf ball 202 having a hardness Hs of 95.0 or less has excellent durability. In this respect, the hardness Hs is more preferably equal to or less than 93.0 and particularly preferably equal to or less than 90.0.

The core 204 preferably has a diameter of 36.0 mm or greater but 41.0 mm or less. The core 204 having a diameter of 36.0 mm or greater can achieve excellent resilience performance of the golf ball 202. In this respect, the diameter is more preferably equal to or greater than 36.5 mm and particularly preferably equal to or greater than 37.0 mm. In the golf ball 202 that includes the core 204 having a diameter of 41.0 mm or less, the inner cover 208, the mid cover 210, and the outer cover 212 can have sufficient thicknesses. The golf ball 202 that includes the inner cover 208, the mid cover 210, and the outer cover 212 which have large thicknesses has excellent durability. In this respect, the diameter is more preferably equal to or less than 40.0 mm and particularly preferably equal to or less than 39.0 mm. The core 204 may have a rib on the surface thereof.

In light of feel at impact, the core 204 has an amount of compressive deformation (comp'n) of preferably 3.0 mm or greater and particularly preferably 3.3 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 4.6 mm and particularly preferably equal to or less than 4.3 mm.

For the inner cover 208, a resin composition is suitably used. Examples of the base polymer of the resin composition include ionomer resins, styrene block-containing thermoplastic elastomers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, and thermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. The golf ball 202 that includes the inner cover 208 including an ionomer resin has excellent resilience performance. An ionomer resin and another resin may be used in combination for the inner cover 208. In this case, the principal component of the base polymer is preferably the ionomer resin. Specifically, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

The inner cover 208 can include the ionomer resin described above for the golf ball 2 of the first embodiment. The inner cover 208 can include the styrene block-containing thermoplastic elastomer described above for the golf ball 2 of the first embodiment.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the resin composition of the inner cover 208 in an adequate amount. The inner cover 208 may include powder of a metal with a high specific gravity.

According to the finding by the inventor of the present invention, the inner cover 208 greatly influences the deformation behavior of the golf ball 202 when the golf ball 202 is hit with a middle iron. The golf ball 202 that includes the inner cover 208 having a great hardness Hi has excellent flight performance when being hit with a middle iron. The inner cover 208 having a hardness Hi that is not excessively great can contribute to the feel at impact of the golf ball 202.

In light of flight performance, the hardness Hi of the inner cover 208 is preferably equal to or greater than 68 and more preferably equal to or greater than 71. In light of feel at impact of the golf ball 202, the hardness Hi is preferably equal to or less than 82 and more preferably equal to or less than 79. The hardness Hi is measured with a JIS-C type hardness scale mounted to an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.). For the measurement, a slab that is formed by hot press and that has a thickness of about 2 mm is used. A slab kept at 23° C. for two weeks is used for the measurement. At the measurement, three slabs are stacked. A slab formed from the same resin composition as the resin composition of the inner cover 208 is used.

The inner cover 208 preferably has a thickness of 0.2 mm or greater but 1.2 mm or less. The inner cover 208 having a thickness of 0.2 mm or greater can easily be formed. In this respect, the thickness is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 202 that includes the inner cover 208 having a thickness of 1.2 mm or less has excellent resilience performance. In this respect, the thickness is more preferably equal to or less than 1.0 mm and particularly preferably equal to or less than 0.9 mm.

For forming the inner cover 208, known methods such as injection molding, compression molding, and the like can be used.

For the mid cover 210, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 202 that includes the mid cover 210 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the inner cover 208 can be used for the mid cover 210.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable other resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the inner cover 208 can be used for the mid cover 210.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the mid cover 210 in an adequate amount.

The mid cover 210 preferably has a JIS-C hardness Hm of 68 or greater but 89 or less. The golf ball 202 that includes the mid cover 210 having a hardness Hm of 68 or greater has excellent flight performance upon a shot with a middle iron. In this respect, the hardness Hm is more preferably equal to or greater than 71. The golf ball 202 that includes the mid cover 210 having a hardness Hm of 89 or less has excellent feel at impact. In this respect, the hardness Hm is more preferably equal to or less than 87. The hardness Hm is measured by the same measurement method as that for the hardness Hi.

The mid cover 210 preferably has a thickness of 0.2 mm or greater but 1.2 mm or less. The mid cover 210 having a thickness of 0.2 mm or greater can easily be formed. In this respect, the thickness is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 202 that includes the mid cover 210 having a thickness of 1.2 mm or less has excellent resilience performance. In this respect, the thickness is more preferably equal to or less than 1.0 mm and particularly preferably equal to or less than 0.9 mm.

For forming the mid cover 210, known methods such as injection molding, compression molding, and the like can be used.

For the outer cover 212, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 202 that includes the outer cover 212 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the inner cover 208 can be used for the outer cover 212.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable resin that can be used in combination with an ionomer resin is an ethylene-(meth)acrylic acid copolymer. The copolymer is obtained by a copolymerization reaction of a monomer composition that contains ethylene and (meth)acrylic acid. In the copolymer, some of the carboxyl groups are neutralized with metal ions. The copolymer includes 3% by weight or greater but 25% by weight or less of a (meth)acrylic acid component. An ethylene-(meth)acrylic acid copolymer having a polar functional group is particularly preferred. A specific example of ethylene-(meth)acrylic acid copolymers is trade name “NUCREL” manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.

Another preferable resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the inner cover 208 can be used for the outer cover 212.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the outer cover 212 in an adequate amount.

The outer cover 212 preferably has a JIS-C hardness Ho of 83 or greater but 98 or less. The golf ball 202 that includes the outer cover 212 having a hardness Ho of 83 or greater has excellent resilience performance. The golf ball 202 has excellent flight performance. In this respect, the hardness Ho is more preferably equal to or greater than 86 and particularly preferably equal to or greater than 89. The golf ball 202 that includes the outer cover 212 having a hardness Ho of 98 or less has excellent feel at impact. In this respect, the hardness Ho is more preferably equal to or less than 96 and particularly preferably equal to or less than 94. The hardness Ho is measured by the same measurement method as that for the hardness Hi.

The outer cover 212 preferably has a thickness of 0.2 mm or greater but 1.2 mm or less. The outer cover 212 having a thickness of 0.2 mm or greater can easily be formed. In this respect, the thickness is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 202 that includes the outer cover 212 having a thickness of 1.2 mm or less has excellent resilience performance. In this respect, the thickness is more preferably equal to or less than 1.0 mm and particularly preferably equal to or less than 0.9 mm.

For forming the outer cover 212, known methods such as injection molding, compression molding, and the like can be used. When forming the outer cover 212, the dimples 214 are formed by pimples formed on the cavity face of a mold.

In the golf ball 202, the hardness Ho of the outer cover 212 is greater than the hardness Hi of the inner cover 208. In the golf ball 202 in which the hardness Ho is greater than the hardness Hi, spin can be suppressed. The golf ball 202 has excellent flight performance upon a shot with a middle iron.

In light of flight performance, the difference (Ho−Hi) between the hardness Ho and the hardness Hi is preferably equal to or greater than 5, more preferably equal to or greater than 6, and particularly preferably equal to or greater than 7. The difference (Ho−H1) is preferably equal to or less than 30.

The hardness Ho of the outer cover 212 is preferably greater than the hardness Hm of the mid cover 210. In the golf ball 202 in which the hardness Ho is greater than the hardness Hm, an outer-hard/inner-soft structure can be achieved in the cover 206. When the golf ball 202 is hit with a middle iron, the spin rate is low. The golf ball 202 has excellent flight performance upon a shot with a middle iron. In light of flight performance, the difference (Ho−Hm) between the hardness Ho and the hardness Hm is preferably equal to or greater than 2. The difference (Ho−Hm) is preferably equal to or less than 25.

In the golf ball 202, the hardness Hi of the inner cover 208 is less than the surface hardness Hs of the core 204. In other words, the surface hardness Hs of the core 204 is greater than the hardness Hi of the inner cover 208. When the golf ball 202 is hit with a middle iron, the inner cover 208 absorbs the shock received by the golf ball 202. The golf ball 202 has excellent feel at impact. In light of feel at impact, the difference (Hs−Hi) between the hardness Hs and the hardness Hi is preferably equal to or greater than 1 and particularly preferably equal to or greater than 2. The difference (Hs−Hi) is preferably equal to or less than 15. In the sphere in which the difference (Hs−Hi) is equal to or less than 15, influence of the inner cover 208 on the deformation behavior of the golf ball 202 when the golf ball 202 is hit with a middle iron is appropriately suppressed. In the sphere, the energy loss is low when the golf ball 202 is hit with a middle iron. Furthermore, in the golf ball 202, the flight distance and the spin rate when the golf ball 202 is hit with a middle iron are less varied.

The hardness Hm of the mid cover 210 may be greater than the hardness Hi of the inner cover 208. In this case, an outer-hard/inner-soft structure can be achieved in the cover 206. When the golf ball 202 is hit with a middle iron, the spin rate is low. The golf ball 202 has excellent flight performance upon a shot with a middle iron.

The hardness Hm of the mid cover 210 may be less than the hardness Hi of the inner cover 208. The mid cover 210 absorbs the shock when the golf ball 202 is hit. The golf ball 202 has excellent feel at impact.

In light of achievement of both desired flight performance and desired feel at impact, the difference (Hm−Hi) between the hardness Hm and the hardness Hi is preferably equal to or greater than −5. The difference (Hm−Hi) is preferably equal to or less than 20.

The cover 206 has a total thickness of preferably 2.5 mm or less. The golf ball 202 that includes the cover 206 having a total thickness of 2.5 mm or less has excellent feel at impact. In this respect, the total thickness is particularly preferably equal to or less than 2.4 mm. In light of durability of the golf ball 202, the total thickness is preferably equal to or greater than 0.3 mm, more preferably equal to or greater than 0.5 mm, and particularly preferably equal to or greater than 0.8 mm.

In light of feel at impact, the golf ball 202 has an amount of compressive deformation (comp'n) of preferably 2.8 mm or greater, more preferably 2.9 mm or greater, and particularly preferably 3.0 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 3.6 mm, more preferably equal to or less than 3.5 mm, and particularly preferably equal to or less than 3.4 mm.

Fourth Embodiment

FIG. 7 shows a golf ball 302 according to a fourth embodiment of the present invention. The golf ball 302 includes a spherical core 304 and a cover 306 covering the core 304. The cover 306 includes an inner cover 308, a first mid cover 310 positioned outside the inner cover 308, a second mid cover 312 positioned outside the first mid cover 310, and an outer cover 314 positioned outside the second mid cover 312. On the surface of the outer cover 314, a large number of dimples 316 are formed. Of the surface of the golf ball 302, a part other than the dimples 316 is a land 318. The golf ball 302 includes a paint layer and a mark layer on the external side of the outer cover 314, but these layers are not shown in the drawing.

The golf ball 302 has a diameter of 40 mm or greater but 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is preferably equal to or less than 44 mm and more preferably equal to or less than 42.80 mm. The golf ball 302 has a weight of 40 g or greater but 50 g or less. In light of attainment of great inertia, the weight is preferably equal to or greater than 44 g and more preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is preferably equal to or less than 45.93 g.

FIG. 8 is a line graph showing a hardness distribution of the core 304 of the golf ball 302 in FIG. 7. The horizontal axis of the graph indicates the ratio (%) of a distance from the central point of the core 304 to the radius of the core 304. The vertical axis of the graph indicates a JIS-C hardness. Nine measuring points obtained by dividing a region from the central point of the core 304 to the surface of the core 304 at intervals of 12.5% of the radius of the core 304 are plotted in the graph. The ratio of the distance from the central point of the core 304 to each of these measuring points to the radius of the core 304 is as follows.

First point: 0.0% (central point)

Second point: 12.5%

Third point: 25.0%

Fourth point: 37.5%

Fifth point: 50.0%

Sixth point: 62.5%

Seventh point: 75.0%

Eighth point: 87.5%

Ninth point: 100.0% (surface)

Hardnesses at the first to eighth points are measured by pressing a JIS-C type hardness scale against a cut plane of the core 304 that has been cut into two halves. A hardness Hs at the ninth point is measured by pressing the JIS-C type hardness scale against the surface of the spherical core 304. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used. In the present invention, a JIS-C hardness at a measuring point whose distance from the central point of the core 304 is x (%) is represented by H(x). The hardness at the central point of the core 304 is represented by H(0).

FIG. 8 also shows a linear approximation curve obtained by a least-square method on the basis of the distances and the hardnesses of the nine measuring points. As is clear from FIG. 8, the broken line does not greatly deviate from the linear approximation curve. In other words, the broken line has a shape close to the linear approximation curve. In the core 304, the hardness linearly increases from its central point toward its surface. When the core 304 is hit with a middle iron, the energy loss is low. The core 304 has excellent resilience performance. When the golf ball 302 is hit with a middle iron, the flight distance is large.

In the core 304, R² of the linear approximation curve obtained by the least-square method is equal to or greater than 0.95. R² is an index indicating the linearity of the broken line. For the core 304 for which R² is equal to or greater than 0.95, the shape of the broken line of the hardness distribution is close to a straight line. The core 304 for which R² is equal to or greater than 0.95 has excellent resilience performance. R² is more preferably equal to or greater than 0.96 and particularly preferably equal to or greater than 0.97. R² is calculated by squaring a correlation coefficient R. The correlation coefficient R is calculated by dividing the covariance of the distance (%) from the central point and the hardness (JIS-C) by the standard deviation of the distance (%) from the central point and the standard deviation of the hardness (JIS-C).

The core 304 is obtained by crosslinking a rubber composition. The rubber composition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) an acid and/or a salt.

During heating and forming of the core 304, the base rubber (a) is crosslinked by the co-crosslinking agent (b). The heat of the crosslinking reaction remains near the central point of the core 304. Thus, during heating and forming of the core 304, the temperature at the central portion is high. The temperature gradually decreases from the central point toward the surface. It is inferred that in the rubber composition, the acid reacts with the metal salt of the co-crosslinking agent (b) to bond to cation. It is inferred that in the rubber composition, the salt reacts with the metal salt of the co-crosslinking agent (b) to exchange cation. By the bonding and the exchange, metal crosslinks are broken. This breaking is likely to occur in the central portion of the core 304 where the temperature is high, and is unlikely to occur near the surface of the core 304. As a result, the crosslinking density of the core 304 increases from its central point toward its surface. In the core 304, an outer-hard/inner-soft structure can be achieved. When the golf ball 302 that includes the core 304 is hit with a middle iron, the spin rate is low. The golf ball 302 achieves excellent flight performance upon a shot with a middle iron.

The rubber composition of the core 304 can include the base rubber (a) described above for the core 4 of the first embodiment.

The rubber composition of the core 304 can include the co-crosslinking agent (b) described above for the core 4 of the first embodiment. Preferable co-crosslinking agents (b) are:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

When the rubber composition includes the α,β-unsaturated carboxylic acid (b1), the rubber composition preferably further includes a metal compound (f). The metal compound (f) reacts with the α,β-unsaturated carboxylic acid (b1) in the rubber composition. A salt obtained by this reaction graft-polymerizes with the molecular chain of the base rubber. The rubber composition of the core 304 can include the metal compound (f) described above for the core 4 of the first embodiment.

In light of resilience performance of the golf ball 302, the amount of the co-crosslinking agent (b) is preferably equal to or greater than 15 parts by weight and particularly preferably equal to or greater than 20 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact, the amount is preferably equal to or less than 50 parts by weight, more preferably equal to or less than 45 parts by weight, and particularly preferably equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber.

The rubber composition of the core 304 can include the crosslinking initiator (c) described above for the core 4 of the first embodiment. In light of resilience performance of the golf ball 302, the amount of the crosslinking initiator (c) is preferably equal to or greater than 0.2 parts by weight and particularly preferably equal to or greater than 0.5 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact and durability of the golf ball 302, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.5 parts by weight, per 100 parts by weight of the base rubber.

The rubber composition of the core 304 can include the acid and/or the salt (d) described above for the core 4 of the first embodiment. In the present invention, the co-crosslinking agent (b) is not included in the concept of the acid and/or the salt (d). As described above, it is inferred that the acid and/or the salt (d) breaks the metal crosslinks by the co-crosslinking agent (b) in the central portion of the core 304 during heating and forming of the core 304.

In light of linearity of the hardness distribution of the core 304, the amount of the acid and/or the salt (d) is preferably equal to or greater than 1 parts by weight, more preferably equal to or greater than 2 parts by weight, even more preferably equal to or greater than 3 parts by weight, and particularly preferably equal to or greater than 4 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably less than 40 parts by weight, more preferably equal to or less than 30 parts by weight, and particularly preferably equal to or less than 20 parts by weight, per 100 parts by weight of the base rubber.

The weight ratio of the co-crosslinking agent (b) and the acid and/or the salt (d) in the rubber composition is preferably equal to or greater than 3/7 but equal to or less than 9/1, and is particularly preferably equal to or greater than 4/6 but equal to or less than 8/2. From the rubber composition in which this weight ratio is within the above range, the core 304 whose hardness linearly increases from its central point toward its surface can be obtained.

As the co-crosslinking agent (b), zinc acrylate is preferably used. Zinc acrylate whose surface is coated with stearic acid or zinc stearate for the purpose of improving dispersibility to rubber is present. In the present invention, when the rubber composition includes this zinc acrylate, this coating material is not included in the concept of the acid and/or the salt (d).

The rubber composition of the core 304 can include the organic sulfur compound (e) described above for the core 4 of the first embodiment. The organic sulfur compound (e) can contribute to control of: the linearity of the hardness distribution of the core 304; and the degree of the outer-hard/inner-soft structure. In light of resilience performance, the amount of the organic sulfur compound (e) is preferably equal to or greater than 0.05 parts by weight and particularly preferably equal to or greater than 0.1 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.0 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler may be included in the core 304. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 304 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, sulfur, a vulcanization accelerator, and the like are added to the rubber composition of the core 304. Crosslinked rubber powder or synthetic resin powder may also be dispersed in the rubber composition.

In the core 304, the difference (Hs−H(0)) between the surface hardness Hs and the central hardness H(0) is preferably equal to or greater than 15. The difference is great. In other words, the core 304 has an outer-hard/inner-soft structure. When the core 304 is hit with a middle iron, the recoil (torsional return) is great, and thus spin is suppressed. The core 304 contributes to the flight performance of the golf ball 302. In light of flight performance, the difference (Hs−H(0)) is more preferably equal to or greater than 20 and particularly preferably equal to or greater than 25. From the standpoint that the core 304 can easily be formed, the difference (Hs−H(0)) is preferably equal to or less than 50.

The hardness H(0) at the central point of the core 304 is preferably equal to or greater than 40.0 but equal to or less than 70.0. The golf ball 302 having a hardness H(0) of 40.0 or greater has excellent resilience performance. In this respect, the hardness H(0) is more preferably equal to or greater than 45.0 and particularly preferably equal to or greater than 50.0. In the core 304 having a hardness H(0) of 70.0 or less, an outer-hard/inner-soft structure can be achieved. In the golf ball 302 that includes the core 304, spin can be suppressed. In this respect, the hardness H(0) is more preferably equal to or less than 68.0 and particularly preferably equal to or less than 66.0.

The hardness Hs at the surface of the core 304 is preferably equal to or greater than 76.0 but equal to or less than 95.0. In the core 304 having a hardness Hs of 76.0 or greater, an outer-hard/inner-soft structure can be achieved. In the golf ball 302 that includes the core 304, spin can be suppressed. In this respect, the hardness Hs is more preferably equal to or greater than 78.0 and particularly preferably equal to or greater than 80.0. The golf ball 302 having a hardness Hs of 95.0 or less has excellent durability. In this respect, the hardness Hs is more preferably equal to or less than 93.0 and particularly preferably equal to or less than 90.0.

The core 304 preferably has a diameter of 35.0 mm or greater but 41.0 mm or less. The core 304 having a diameter of 35.0 mm or greater can achieve excellent resilience performance of the golf ball 302. In this respect, the diameter is more preferably equal to or greater than 35.5 mm and particularly preferably equal to or greater than 36.0 mm. In the golf ball 302 that includes the core 304 having a diameter of 41.0 mm or less, the inner cover 308, the first mid cover 310, the second mid cover 312, and the outer cover 314 can have sufficient thicknesses. The golf ball 302 that includes the inner cover 308, the first mid cover 310, the second mid cover 312, and the outer cover 314 which have large thicknesses has excellent durability. In this respect, the diameter is more preferably equal to or less than 40.0 mm and particularly preferably equal to or less than 39.0 mm. The core 304 may have a rib on the surface thereof.

In light of feel at impact, the core 304 has an amount of compressive deformation (comp'n) of preferably 3.0 mm or greater and particularly preferably 3.3 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 4.6 mm and particularly preferably equal to or less than 4.3 mm.

For the inner cover 308, a resin composition is suitably used. Examples of the base polymer of the resin composition include ionomer resins, styrene block-containing thermoplastic elastomers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, and thermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. The golf ball 302 that includes the inner cover 308 including an ionomer resin has excellent resilience performance. An ionomer resin and another resin may be used in combination for the inner cover 308. In this case, the principal component of the base polymer is preferably the ionomer resin. Specifically, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

The inner cover 308 can include the ionomer resin described above for the golf ball 2 of the first embodiment. The inner cover 308 can include the styrene block-containing thermoplastic elastomer described above for the golf ball 2 of the first embodiment.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the resin composition of the inner cover 308 in an adequate amount. The inner cover 308 may include powder of a metal with a high specific gravity.

According to the finding by the inventor of the present invention, the inner cover 308 greatly influences the deformation behavior of the golf ball 302 when the golf ball 302 is hit with a middle iron. The golf ball 302 that includes the inner cover 308 having a great hardness Hi has excellent flight performance when being hit with a middle iron. The inner cover 308 having a hardness H1 that is not excessively great can contribute to the feel at impact of the golf ball 302.

In light of flight performance, the hardness Hi of the inner cover 308 is preferably equal to or greater than 68 and more preferably equal to or greater than 71. In light of feel at impact of the golf ball 302, the hardness Hi is preferably equal to or less than 82 and more preferably equal to or less than 79. The hardness Hi is measured with a JIS-C type hardness scale mounted to an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.). For the measurement, a slab that is formed by hot press and that has a thickness of about 2 mm is used. A slab kept at 23° C. for two weeks is used for the measurement. At the measurement, three slabs are stacked. A slab formed from the same resin composition as the resin composition of the inner cover 308 is used.

The inner cover 308 preferably has a thickness Ti of 0.2 mm or greater but 1.8 mm or less. The inner cover 308 having a thickness Ti of 0.2 mm or greater can easily be formed. In this respect, the thickness Ti is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 302 that includes the inner cover 308 having a thickness Ti of 1.8 mm or less has excellent resilience performance. In this respect, the thickness Ti is more preferably equal to or less than 1.6 mm and particularly preferably equal to or less than 1.4 mm.

For forming the inner cover 308, known methods such as injection molding, compression molding, and the like can be used.

For the first mid cover 310, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 302 that includes the first mid cover 310 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the inner cover 308 can be used for the first mid cover 310.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable other resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the inner cover 308 can be used for the first mid cover 310.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the first mid cover 310 in an adequate amount.

The first mid cover 310 preferably has a JIS-C hardness Hm1 of 68 or greater but 89 or less. The golf ball 302 that includes the first mid cover 310 having a hardness Hm1 of 68 or greater has excellent flight performance upon a shot with a middle iron. In this respect, the hardness Hm1 is more preferably equal to or greater than 71. The golf ball 302 that includes the first mid cover 310 having a hardness Hm1 of 89 or less has excellent feel at impact. In this respect, the hardness Hm1 is more preferably equal to or less than 87. The hardness Hm1 is measured by the same measurement method as that for the hardness Hi.

The first mid cover 310 preferably has a thickness Tm1 of 0.2 mm or greater but 1.8 mm or less. The first mid cover 310 having a thickness Tm1 of 0.2 mm or greater can easily be formed. In this respect, the thickness Tm1 is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 302 that includes the first mid cover 310 having a thickness Tm1 of 1.8 mm or less has excellent resilience performance. In this respect, the thickness Tm1 is more preferably equal to or less than 1.6 mm and particularly preferably equal to or less than 1.4 mm.

For forming the first mid cover 310, known methods such as injection molding, compression molding, and the like can be used.

For the second mid cover 312, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 302 that includes the second mid cover 312 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the inner cover 308 can be used for the second mid cover 312.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable other resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the inner cover 308 can be used for the second mid cover 312.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the second mid cover 312 in an adequate amount.

The second mid cover 312 preferably has a JIS-C hardness Hm2 of 68 or greater but 89 or less. The golf ball 302 that includes the second mid cover 312 having a hardness Hm2 of 68 or greater has excellent flight performance upon a shot with a middle iron. In this respect, the hardness Hm2 is more preferably equal to or greater than 71. The golf ball 302 that includes the second mid cover 312 having a hardness Hm2 of 89 or less has excellent feel at impact. In this respect, the hardness Hm2 is more preferably equal to or less than 87. The hardness Hm2 is measured by the same measurement method as that for the hardness Hi.

The second mid cover 312 preferably has a thickness Tm2 of 0.2 mm or greater but 1.8 mm or less. The second mid cover 312 having a thickness Tm2 of 0.2 mm or greater can easily be formed. In this respect, the thickness Tm2 is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 302 that includes the second mid cover 312 having a thickness Tm2 of 1.8 mm or less has excellent resilience performance. In this respect, the thickness Tm2 is more preferably equal to or less than 1.6 mm and particularly preferably equal to or less than 1.4 mm.

For forming the second mid cover 312, known methods such as injection molding, compression molding, and the like can be used.

For the outer cover 314, a resin composition is suitably used. A preferable base polymer of the resin composition is an ionomer resin. The golf ball 302 that includes the outer cover 314 including the ionomer resin has excellent resilience performance. The ionomer resin described above for the inner cover 308 can be used for the outer cover 314.

An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

A preferable resin that can be used in combination with an ionomer resin is an ethylene-(meth)acrylic acid copolymer. The copolymer is obtained by a copolymerization reaction of a monomer composition that contains ethylene and (meth)acrylic acid. In the copolymer, some of the carboxyl groups are neutralized with metal ions. The copolymer includes 3% by weight or greater but 25% by weight or less of a (meth)acrylic acid component. An ethylene-(meth)acrylic acid copolymer having a polar functional group is particularly preferred. A specific example of ethylene-(meth)acrylic acid copolymers is trade name “NUCREL” manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.

Another preferable resin that can be used in combination with an ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above for the inner cover 308 can be used for the outer cover 314.

According to need, a coloring agent such as titanium dioxide and a fluorescent pigment, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the outer cover 314 in an adequate amount.

The outer cover 314 preferably has a JIS-C hardness Ho of 83 or greater but 98 or less. The golf ball 302 that includes the outer cover 314 having a hardness Ho of 83 or greater has excellent resilience performance. The golf ball 302 has excellent flight performance. In this respect, the hardness Ho is more preferably equal to or greater than 86 and particularly preferably equal to or greater than 89. The golf ball 302 that includes the outer cover 314 having a hardness Ho of 98 or less has excellent feel at impact. In this respect, the hardness Ho is more preferably equal to or less than 96 and particularly preferably equal to or less than 94. The hardness Ho is measured by the same measurement method as that for the hardness Hi.

The outer cover 314 preferably has a thickness To of 0.2 mm or greater but 1.8 mm or less. The outer cover 314 having a thickness To of 0.2 mm or greater can easily be formed. In this respect, the thickness To is more preferably equal to or greater than 0.4 mm and particularly preferably equal to or greater than 0.6 mm. The golf ball 302 that includes the outer cover 314 having a thickness To of 1.8 mm or less has excellent resilience performance. In this respect, the thickness To is more preferably equal to or less than 1.6 mm and particularly preferably equal to or less than 1.4 mm.

For forming the outer cover 314, known methods such as injection molding, compression molding, and the like can be used. When forming the outer cover 314, the dimples 316 are formed by pimples formed on the cavity face of a mold.

In the golf ball 302, the hardness Ho of the outer cover 314 is greater than the hardness Hi of the inner cover 308. In the golf ball 302 in which the hardness Ho is greater than the hardness Hi, spin can be suppressed. The golf ball 302 has excellent flight performance upon a shot with a middle iron.

In light of flight performance, the difference (Ho−Hi) between the hardness Ho and the hardness Hi is preferably equal to or greater than 5, more preferably equal to or greater than 6, and particularly preferably equal to or greater than 7. The difference (Ho−Hi) is preferably equal to or less than 30.

The hardness Ho of the outer cover 314 is preferably greater than the hardness Hm1 of the first mid cover 310. In the golf ball 302 in which the hardness Ho is greater than the hardness Hm1, an outer-hard/inner-soft structure can be achieved in the cover 306. When the golf ball 302 is hit with a middle iron, the spin rate is low. The golf ball 302 has excellent flight performance upon a shot with a middle iron. In light of flight performance, the difference (Ho−Hm1) between the hardness Ho and the hardness Hm1 is preferably equal to or greater than 2. The difference (Ho−Hm1) is preferably equal to or less than 25.

The hardness Ho of the outer cover 314 is preferably greater than the hardness Hm2 of the second mid cover 312. In the golf ball 302 in which the hardness Ho is greater than the hardness Hm2, an outer-hard/inner-soft structure can be achieved in the cover 306. When the golf ball 302 is hit with a middle iron, the spin rate is low. The golf ball 302 has excellent flight performance upon a shot with a middle iron. In light of flight performance, the difference (Ho−Hm2) between the hardness Ho and the hardness Hm2 is preferably equal to or greater than 2. The difference (Ho−Hm2) is preferably equal to or less than 25.

In the golf ball 302, the hardness Hi of the inner cover 308 is less than the surface hardness Hs of the core 304. In other words, the surface hardness Hs of the core 304 is greater than the hardness Hi of the inner cover 308. When the golf ball 302 is hit with a middle iron, the inner cover 308 absorbs the shock received by the golf ball 302. The golf ball 302 has excellent feel at impact. In light of feel at impact, the difference (Hs−Hi) between the hardness Hs and the hardness Hi is preferably equal to or greater than 1 and particularly preferably equal to or greater than 2. The difference (Hs−Hi) is preferably equal to or less than 15. In the sphere in which the difference (Hs−Hi) is equal to or less than 15, influence of the inner cover 308 on the deformation behavior of the golf ball 302 when the golf ball 302 is hit with a middle iron is appropriately suppressed. In the sphere, the energy loss is low when the golf ball 302 is hit with a middle iron. Furthermore, in the golf ball 302, the flight distance and the spin rate when the golf ball 302 is hit with a middle iron are less varied.

The hardness Hm1 of the first mid cover 310 may be greater than the hardness Hi of the inner cover 308. In this case, an outer-hard/inner-soft structure can be achieved in the cover 306. When the golf ball 302 is hit with a middle iron, the spin rate is low. The golf ball 302 has excellent flight performance upon a shot with a middle iron.

The hardness Hm1 of the first mid cover 310 may be less than the hardness Hi of the inner cover 308. The first mid cover 310 absorbs the shock when the golf ball 302 is hit. The golf ball 302 has excellent feel at impact.

In light of achievement of both desired flight performance and desired feel at impact, the difference (Hm1−Hi) between the hardness Hm1 and the hardness Hi is preferably equal to or greater than −8. The difference (Hm1−Hi) is preferably equal to or less than 10.

In the golf ball 302, the hardness Hm2 of the second mid cover 312 may be greater than the hardness Hm1 of the first mid cover 310. In this case, an outer-hard/inner-soft structure can be achieved in the cover 306. When the golf ball 302 is hit with a middle iron, the spin rate is low. The golf ball 302 has excellent flight performance upon a shot with a middle iron.

The hardness Hm2 of the second mid cover 312 may be less than the hardness Hm1 of the first mid cover 310. In this case, the second mid cover 312 absorbs the shock when the golf ball 302 is hit. The golf ball 302 has excellent feel at impact.

In light of achievement of both desired flight performance and desired feel at impact, the difference (Hm2−Hm1) between the hardness Hm2 and the hardness Hm1 is preferably equal to or greater than −15. The difference (Hm2−Hm1) is preferably equal to or less than 15.

The cover 306 has a total thickness of preferably 6.0 mm or less. The golf ball 302 that includes the cover 306 having a total thickness of 6.0 mm or less has excellent feel at impact. In this respect, the total thickness is particularly preferably equal to or less than 5.6 mm. In light of durability of the golf ball 302, the total thickness is preferably equal to or greater than 0.6 mm, more preferably equal to or greater than 0.8 mm, and particularly preferably equal to or greater than 1.0 mm.

In light of feel at impact, the golf ball 302 has an amount of compressive deformation (comp'n) of preferably 2.8 mm or greater, more preferably 2.9 mm or greater, and particularly preferably 3.0 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 3.6 mm, more preferably equal to or less than 3.5 mm, and particularly preferably equal to or less than 3.4 mm.

EXAMPLES Experiment 1 Example I-1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 27 parts by weight of zinc diacrylate (trade name “Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.2 parts by weight of 2-naphthalenethiol, 10 parts by weight of zinc stearate, and 0.75 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 25 minutes to obtain a core with a diameter of 37.9 mm. The amount of barium sulfate was adjusted such that the weight of a golf ball is 45.4 g.

A resin composition was obtained by kneading 24 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 50 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 26 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The core was placed into a mold. The resin composition was injected around the core by injection molding to form an inner cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 40 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 20 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The sphere consisting of the core and the inner cover was placed into a mold. The resin composition was injected around the sphere by injection molding to form a mid cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 5 parts by weight of an ionomer resin (the aforementioned“Himilan AM7337”), 10 parts by weight of another ionomer resin (the aforementioned “Himilan 1555”), 55 parts by weight of still another ionomer resin (the aforementioned “Himilan AM7329”), 30 parts by weight of an ethylene-(meth)acrylic acid copolymer (trade name “NUCREL N1050H”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 3 parts by weight of titanium dioxide, and 0.2 parts by weight of an ultraviolet absorber (trade name “TINUVIN 770”, manufactured by Ciba Japan K.K.) with a twin-screw kneading extruder. The sphere consisting of the core, the inner cover, and the mid cover was placed into a final mold having a large number of pimples on its cavity face. The resin composition was injected around the sphere by injection molding to form an outer cover with a thickness of 0.8 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the outer cover. A clear paint including a two-component curing type polyurethane as a base material was applied to the outer cover to obtain a golf ball of Example I-1 with a diameter of 42.7 mm.

Examples I-2 to I-14 and Comparative Examples I-1 to I-9

Golf balls of Examples I-2 to I-14 and Comparative Examples I-1 to I-9 were obtained in the same manner as Example I-1, except the specifications of the core, the inner cover, the mid cover, and the outer cover were as shown in Tables I-10 to I-14 below. The composition of the core is shown in detail in Tables I-1 and I-2 below. The compositions of the inner cover, the mid cover, and the outer cover are shown in detail in Tables I-3 and I-4 below. A hardness distribution of the core is shown in Tables I-5 to I-9 below. The golf ball according to Comparative Example I-7 does not have a mid cover.

[Hit with Middle Iron (I#5)]

A 5-iron (trade name “XXIO”, manufactured by DUNLOP SPORTS CO. LTD., shaft hardness: R, loft angle: 24°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the condition of a head speed of 34 m/sec. The spin rate was measured immediately after the hit. Furthermore, the distance from the launch point to the stop point was measured. The average value of data obtained by 12 measurements is shown in Tables I-10 to I-14 below.

TABLE I-1 Composition of Core (parts by weight) A B C D E F BR-730 100 100 100 100 100 100 Sanceler SR 26.5 27.0 27.5 29.5 31.5 25.5 Zinc oxide 5.0 5.0 5.0 5.0 5.0 5.0 Barium sulfate * * * * * * 2-naphthalene- 0.2 0.2 0.2 0.2 0.2 0.2 thiol Zinc octoate — — — — — 2.5 Zinc laurate — — — — — — Zinc myristate — — — — — — Zinc stearate 0.5 10.0 20.0 30.0 40.0 — Dicumyl 0.75 0.75 0.75 0.75 0.75 0.75 peroxide * Appropriate amount

TABLE I-2 Composition of Core (parts by weight) G H I J K BR-730 100 100 100 100 100 Sanceler SR 25.0 25.0 26.0 25.5 27.0 Zinc oxide 5.0 5.0 5.0 5.0 5.0 Barium sulfate * * * * * 2-naphthalenethiol 0.2 0.2 0.2 0.2 0.2 Zinc octoate 5.0 — — — — Zinc laurate — 10.0 — — — Zinc myristate — — 5.0 10.0 — Zinc stearate — — — — — Dicumyl peroxide 0.75 0.75 0.75 0.75 0.75 * Appropriate amount

The details of the compounds listed in Tables I-1 and I-2 are as follows.

BR-730: a high-cis polybutadiene manufactured by JSR Corporation (cis-1,4-bond content: 96% by weight, 1,2-vinyl bond content: 1.3% by weight, Mooney viscosity (ML₁₊₄(100° C.)): 55, molecular weight distribution (Mw/Mn): 3)

Sanceler SR: zinc diacrylate manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD. (a product coated with 10% by weight of stearic acid)

Zinc oxide: trade name “Ginrei R” manufactured by Toho Zinc Co., Ltd.

Barium sulfate: trade name “Barium Sulfate BD” manufactured by Sakai Chemical Industry Co., Ltd.

2-naphthalenethiol: a product of Tokyo Chemical Industry Co., Ltd.

Zinc octoate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc laurate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc myristate: a product of NOF Corporation

Zinc stearate: a product of Wako Pure Chemical Industries, Ltd.

Dicumyl peroxide: trade name “Percumyl D” manufactured by NOF Corporation

TABLE I-3 Composition of Cover (parts by weight) C1 C2 C3 C4 Himilan AM7337 51 45 40 24 Himilan 1555 — — — — Himilan AM7329 40 40 40 50 NUCREL N1050H — — — — Rabalon T3221C  9 15 20 26 Titanium dioxide (A220)  6  6  6  6 TINUVIN 770 — — — — Hardness (JIS-C) 89 87 85 83 Hardness (Shore D) 58 56 54 52

TABLE I-4 Composition of Cover (parts by weight) C5 C6 C7 C8 Himilan AM7337 30 26 5 53 Himilan 1555 — — 10 — Himilan AM7329 40 40 55 27 NUCREL N1050H — — 30 — Rabalon T3221C 30 34 — 20 Titanium dioxide (A220)  6  6 3 3 TINUVIN 770 — — 0.2 0.2 Hardness (JIS-C) 79 76 92 81 Hardness (Shore D) 48 45 61 50

TABLE I-5 Hardness Distribution of Core (JIS-C) Ex. Ex. Ex. Ex. Ex. I-1 I-2 I-3 I-4 I-5 H(0.0) 54.0 54.0 54.0 54.0 54.0 H(12.5) 59.7 59.7 59.7 59.7 59.7 H(25.0) 62.9 62.9 62.9 62.9 62.9 H(37.5) 64.2 64.2 64.2 64.2 64.2 H(50.0) 66.6 66.6 66.6 66.6 66.6 H(62.5) 71.4 71.4 71.4 71.4 71.4 H(75.0) 75.4 75.4 75.4 75.4 75.4 H(87.5) 78.7 78.7 78.7 78.7 78.7 Hs 82.1 82.1 82.1 82.1 82.1 Hs − H(0) 28.1 28.1 28.1 28.1 28.1 Diameter (mm) 37.9 37.9 37.9 37.9 37.9 Comp'n 3.85 3.85 3.85 3.85 3.85

TABLE I-6 Hardness Distribution of Core (JIS-C) Ex. Ex. Ex. Ex. Ex. I-6 I-7 I-8 I-9 I-10 H(0.0) 53.6 51.4 54.2 54.9 53.3 H(12.5) 58.2 57.5 58.1 58.9 58.3 H(25.0) 61.6 61.1 62.0 63.5 62.5 H(37.5) 64.9 63.4 64.1 66.8 65.3 H(50.0) 67.1 67.5 66.0 68.2 67.2 H(62.5) 70.6 73.4 70.6 69.7 71.5 H(75.0) 74.8 77.3 76.1 76.2 76.9 H(87.5) 79.9 81.2 79.7 79.6 80.4 Hs 83.6 84.2 82.8 82.6 83.7 Hs − H(0) 30.0 32.8 28.6 27.7 30.4 Diameter (mm) 37.9 37.9 37.9 37.9 37.9 Comp'n 3.87 3.83 3.85 3.86 3.84

TABLE I-7 Hardness Distribution of Core (JIS-C) Comp. Ex. Ex. Ex. Ex. Ex. I-11 I-12 I-13 I-14 I-1 H(0.0) 56.5 59.2 54.0 57.5 61.9 H(12.5) 59.6 61.4 59.6 63.4 63.1 H(25.0) 61.9 63.1 62.8 66.5 64.2 H(37.5) 62.5 63.7 64.0 68.5 64.2 H(50.0) 66.3 66.5 66.4 69.6 66.8 H(62.5) 73.4 70.6 71.2 70.8 70.3 H(75.0) 75.0 71.8 75.2 74.3 70.4 H(87.5) 77.8 72.6 78.4 78.1 68.4 Hs 80.9 78.4 81.6 82.2 70.2 Hs − H(0) 24.4 19.2 27.6 24.7 8.3 Diameter (mm) 37.9 37.9 37.1 37.9 37.9 Comp'n 3.86 3.85 3.85 3.86 3.86

TABLE I-8 Hardness Distribution of Core (JIS-C) Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. I-2 I-3 I-4 I-5 H(0.0) 54.0 54.0 54.0 54.0 H(12.5) 59.7 59.7 59.7 59.7 H(25.0) 62.9 62.9 62.9 62.9 H(37.5) 64.2 64.2 64.2 64.2 H(50.0) 66.6 66.6 66.6 66.6 H(62.5) 71.4 71.4 71.4 71.4 H(75.0) 75.4 75.4 75.4 75.4 H(87.5) 78.7 78.7 78.7 78.7 Hs 82.1 82.1 82.1 82.1 Hs − H(0) 28.1 28.1 28.1 28.1 Diameter (mm) 37.9 37.9 37.9 37.9 Comp'n 3.85 3.85 3.85 3.85

TABLE I-9 Hardness Distribution of Core (JIS-C) Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. I-6 I-7 I-8 I-9 H(0.0) 54.0 54.0 59.0 54.0 H(12.5) 59.7 59.7 64.4 59.7 H(25.0) 62.9 62.9 67.0 62.9 H(37.5) 64.2 64.2 67.4 64.2 H(50.0) 66.6 66.6 68.3 66.6 H(62.5) 71.4 71.4 70.2 71.4 H(75.0) 75.4 75.4 73.6 75.4 H(87.5) 78.7 78.7 78.2 78.7 Hs 82.1 82.1 82.3 82.1 Hs − H(0) 28.1 28.1 23.3 28.1 Diameter (mm) 37.9 37.9 37.9 37.9 Comp'n 3.85 3.85 3.86 3.85

TABLE I-10 Results of Evaluation Ex. Ex. Ex. Ex. Ex. I-1 I-2 I-3 I-4 I-5 Core Composition B B B B B Acid/salt (PHR) 10.0 10.0 10.0 10.0 10.0 R² 0.99 0.99 0.99 0.99 0.99 Hs − H(0) 28.1 28.1 28.1 28.1 28.1 Diameter (mm) 37.9 37.9 37.9 37.9 37.9 Comp'n 3.85 3.85 3.85 3.85 3.85 Inner cover Composition C4 C3 C2 C3 C2 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hi (JIS-C) 83 85 87 85 87 Mid cover Composition C3 C2 C3 C6 C1 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hm (JIS-C) 85 87 85 76 89 Outer cover Composition C7 C7 C7 C7 C7 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho (JIS-C) 92 92 92 92 92 Ball Hi − Hs 0.9 2.9 4.9 2.9 4.9 Ho − Hi 9 7 5 7 5 Cover thickness (mm) 2.4 2.4 2.4 2.4 2.4 Comp'n 3.23 3.22 3.22 3.25 3.21 Spin (rpm) 4031 3899 3869 4038 3797 Flight distance (m) 145.2 146.2 146.4 145.1 147.0

TABLE I-11 Results of Evaluation Ex. Ex. Ex. Ex. Ex. I-6 I-7 I-8 I-9 I-10 Core Composition F G H I J Acid/salt (PHR) 2.5 5.0 10.0 5.0 10.0 R² 0.99 0.99 0.99 0.98 0.99 Hs − H(0) 30 32.8 28.6 27.7 30.4 Diameter (mm) 37.9 37.9 37.9 37.9 37.9 Comp'n 3.87 3.83 3.85 3.86 3.84 Inner cover Composition C3 C3 C3 C3 C3 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hi (JIS-C) 85 85 85 85 85 Mid cover Composition C2 C2 C2 C2 C2 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hm (JIS-C) 87 87 87 87 87 Outer cover Composition C7 C7 C7 C7 C7 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho (JIS-C) 92 92 92 92 92 Ball Hi − Hs 1.4 0.8 2.2 2.4 1.3 Ho − Hi 7 7 7 7 7 Cover thickness (mm) 2.4 2.4 2.4 2.4 2.4 Comp'n 3.24 3.2 3.22 3.23 3.21 Spin (rpm) 3926 3879 3976 3991 3934 Flight distance (m) 146.0 146.4 145.6 145.6 145.9

TABLE I-12 Results of Evaluation Comp. Ex. Ex. Ex. Ex. Ex. I-11 I-12 I-13 I-14 I-1 Core Composition C D B A E Acid/salt (PHR) 20.0 30.0 10.0 0.5 40.0 R² 0.97 0.96 0.99 0.96 0.84 Hs − H(0) 24.4 19.2 27.6 24.7 8.3 Diameter (mm) 37.9 37.9 37.1 37.9 37.9 Comp'n 3.86 3.85 3.85 3.86 3.86 Inner cover Composition C3 C3 C4 C4 C4 Thickness (mm) 0.8 0.8 1.0 0.8 0.8 Hardness Hi (JIS-C) 85 85 83 83 83 Mid cover Composition C2 C2 C3 C3 C3 Thickness (mm) 0.8 0.8 1.0 0.8 0.8 Hardness Hm (JIS-C) 87 87 85 85 85 Outer cover Composition C7 C7 C7 C7 C7 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho (JIS-C) 92 92 92 92 92 Ball Hi − Hs 4.1 6.6 1.4 0.8 12.8 Ho − Hi 7 7 9 9 9 Cover thickness (mm) 2.4 2.4 2.8 2.4 2.4 Comp'n 3.23 3.22 3.23 3.23 3.23 Spin (rpm) 3996 4012 4036 4022 4145 Flight distance (m) 145.5 145.4 145.1 145.2 144.1

TABLE I-13 Results of Evaluation Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. I-2 I-3 I-4 I-5 Core Composition B B B B Acid/salt (PHR) 10.0 10.0 10.0 10.0 R² 0.99 0.99 0.99 0.99 Hs − H(0) 28.1 28.1 28.1 28.1 Diameter (mm) 37.9 37.9 37.9 37.9 Comp'n 3.85 3.85 3.85 3.85 Inner cover Composition C6 C5 C6 C5 Thickness (mm) 0.8 0.8 0.8 0.8 Hardness Hi (JIS-C) 76 79 76 79 Mid cover Composition C3 C3 C1 C6 Thickness (mm) 0.8 0.8 0.8 0.8 Hardness Hm (JIS-C) 85 85 89 76 Outer cover Composition C7 C7 C7 C7 Thickness (mm) 0.8 0.8 0.8 0.8 Hardness Ho (JIS-C) 92 92 92 92 Ball Hi − Hs −6.1 −3.1 −6.1 −3.1 Ho − Hi 16 13 16 13 Cover thickness (mm) 2.4 2.4 2.4 2.4 Comp'n 3.25 3.25 3.25 3.31 Spin (rpm) 4193 4112 4121 4220 Flight distance (m) 143.9 144.5 144.4 143.6

TABLE I-14 Results of Evaluation Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. I-6 I-7 I-8 I-9 Core Composition B B K B Acid/salt (PHR) 10.0 10.0 0.0 10.0 R² 0.99 0.99 0.94 0.99 Hs − H(0) 28.1 28.1 23.3 28.1 Diameter (mm) 37.9 37.9 37.9 37.9 Comp'n 3.85 3.85 3.86 3.85 Inner cover Composition C6 C4 C4 C4 Thickness (mm) 0.8 1.2 0.8 0.8 Hardness Hi (JIS-C) 76 83 83 83 Mid cover Composition C5 — C3 C3 Thickness (mm) 0.8 — 0.8 0.8 Hardness Hm (JIS-C) 79 — 85 85 Outer cover Composition C7 C7 C7 C8 Thickness (mm) 0.8 1.2 0.8 0.8 Hardness Ho (JIS-C) 92 92 92 81 Ball Hi − Hs −6 0.9 0.7 0.9 Ho − Hi 16 9 9 −2 Cover thickness (mm) 2.4 2.4 2.4 2.4 Comp'n 3.31 3.22 3.23 3.26 Spin (rpm) 4265 4088 4065 4091 Flight distance (m) 143.3 144.7 144.8 144.6

As shown in Tables I-10 to I-14, the golf balls according to Examples have excellent flight performance upon a shot with a middle iron. From the results of evaluation, advantages of the present invention are clear.

Experiment 2 Example II-1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 27.0 parts by weight of zinc diacrylate (trade name “Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5.0 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.2 parts by weight of 2-naphthalenethiol, 10.0 parts by weight of zinc stearate, and 0.75 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 25 minutes to obtain a core with a diameter of 36.3 mm. The amount of barium sulfate was adjusted such that the weight of a golf ball is 45.4 g.

A resin composition was obtained by kneading 40 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 20 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The core was placed into a mold. The resin composition was injected around the core by injection molding to form a first cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 45 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 15 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The sphere consisting of the core and the first cover was placed into a mold. The resin composition was injected around the sphere by injection molding to form a second cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 51 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 9 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The sphere consisting of the core, the first cover, and the second cover was placed into a mold. The resin composition was injected around the sphere by injection molding to form a third cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 5 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 10 parts by weight of another ionomer resin (the aforementioned “Himilan 1555”), 55 parts by weight of still another ionomer resin (the aforementioned “Himilan AM7329”), 30 parts by weight of an ethylene-(meth)acrylic acid copolymer (trade name “NUCREL N1050H”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 3 parts by weight of titanium dioxide, and 0.2 parts by weight of an ultraviolet absorber (trade name “TINUVIN 770”, manufactured by Ciba Japan K.K.) with a twin-screw kneading extruder. The sphere consisting of the core, the first cover, the second cover, and the third cover was placed into a final mold having a large number of pimples on its cavity face. The resin composition was injected around the sphere by injection molding to form a fourth cover with a thickness of 0.8 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the fourth cover. A clear paint including a two-component curing type polyurethane as a base material was applied to the fourth cover to obtain a golf ball of Example II-1 with a diameter of 42.7 mm.

Examples II-2 to II-14 and Comparative Examples II-1 to II-10

Golf balls of Examples II-2 to II-14 and Comparative Examples II-1 to II-10 were obtained in the same manner as Example II-1, except the specifications of the core, the first cover, the second cover, the third cover, and the fourth cover were as shown in Tables II-9 to II-13 below. The composition of the core is shown in detail in Tables II-1 and II-2 below. The composition of the cover is shown in detail in Table II-3 below. A hardness distribution of the core is shown in Tables II-4 to II-8 below. The golf ball according to Comparative Example II-5 does not have a second cover and a third cover. The golf ball according to Comparative Example II-6 does not have a third cover. The golf ball according to Comparative Example II-7 does not have a third cover.

[Hit with Middle Iron (I#5)]

A 5-iron (trade name “XXIO”, manufactured by DUNLOP SPORTS CO. LTD., shaft hardness: R, loft angle: 24°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the condition of a head speed of 34 m/sec. The spin rate was measured immediately after the hit. Furthermore, the distance from the launch point to the stop point was measured. The average value of data obtained by 12 measurements is shown in Tables II-9 to II-13 below.

TABLE II-1 Composition of Core (parts by weight) A B C D E F BR-730 100 100 100 100 100 100 Sanceler SR 27.0 25.5 25.0 25.0 26.0 25.5 Zinc oxide 5.0 5.0 5.0 5.0 5.0 5.0 Barium sulfate * * * * * * 2-naphthalene- 0.2 0.2 0.2 0.2 0.2 0.2 thiol Zinc octoate — 2.5 5.0 — — — Zinc laurate — — — 10.0 — — Zinc myristate — — — — 5.0 10.0 Zinc stearate 10.0 — — — — — Dicumyl 0.75 0.75 0.75 0.75 0.75 0.75 peroxide * Appropriate amount

TABLE II-2 Composition of Core (parts by weight) G H I J K BR-730 100 100 100 100 100 Sanceler SR 27.5 29.5 27.0 26.5 31.5 Zinc oxide 5.0 5.0 5.0 5.0 5.0 Barium sulfate * * * * * 2-naphthalenethiol 0.2 0.2 0.2 0.2 0.2 Zinc octoate — — — — — Zinc laurate — — — — — Zinc myristate — — — — — Zinc stearate 20.0 30.0 — 0.5 40.0 Dicumyl peroxide 0.75 0.75 0.75 0.75 0.75 * Appropriate amount

The details of the compounds listed in Tables II-1 and II-2 are as follows.

BR-730: a high-cis polybutadiene manufactured by JSR Corporation (cis-1,4-bond content: 96% by weight, 1,2-vinyl bond content: 1.3% by weight, Mooney viscosity (ML₁₊₄(100° C.)) 55, molecular weight distribution (Mw/Mn): 3)

Sanceler SR: zinc diacrylate manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD. (a product coated with 10% by weight of stearic acid)

Zinc oxide: trade name “Ginrei R” manufactured by Toho Zinc Co., Ltd.

Barium sulfate: trade name “Barium Sulfate BD” manufactured by Sakai Chemical Industry Co., Ltd.

2-naphthalenethiol: a product of Tokyo Chemical Industry Co., Ltd.

Zinc octoate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc laurate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc myristate: a product of NOF Corporation

Zinc stearate: a product of Wako Pure Chemical Industries, Ltd.

Dicumyl peroxide: trade name “Percumyl D” manufactured by NOF Corporation

TABLE II-3 Composition of Cover (parts by weight) C1 C2 C3 C4 C5 C6 C7 Himilan AM7337 5 51 45 40 24 30 26 Himilan 1555 10 — — — — — — Himilan AM7329 55 40 40 40 50 40 40 NUCRELN 1050H 30 — — — — — — Rabalon T3221C —  9 15 20 26 30 34 Titanium dioxide (A220) 3  6  6  6  6  6  6 TINUVIN 770 0.2 — — — — — — Hardness (JIS-C) 92 89 87 85 83 79 76 Hardness (Shore D) 61 58 56 54 52 48 45

TABLE II-4 Hardness of Core Ex. Ex. Ex. Ex. Ex. II-1 II-2 II-3 II-4 II-5 Composition A A A A A H(0.0) 54.0 54.0 54.0 54.0 54.0 H(12.5) 59.6 59.6 59.6 59.6 59.6 H(25.0) 62.8 62.8 62.8 62.8 62.8 H(37.5) 64.1 64.1 64.1 64.1 64.1 H(50.0) 66.1 66.1 66.1 66.1 66.1 H(62.5) 70.5 70.5 70.5 70.5 70.5 H(75.0) 74.2 74.2 74.2 74.2 74.2 H(87.5) 77.2 77.2 77.2 77.2 77.2 Hs 81.4 81.4 81.4 81.4 81.4 Hs − H(0.0) 27.4 27.4 27.4 27.4 27.4 Diameter (mm) 36.3 36.3 36.3 36.3 36.3

TABLE II-5 Hardness of Core Ex. Ex. Ex. Ex. Ex. II-6 II-7 II-8 II-9 II-10 Composition A B C D E H(0.0) 54.0 53.6 51.4 54.2 54.9 H(12.5) 58.8 58.1 57.5 58.1 58.9 H(25.0) 61.4 61.3 60.9 61.9 63.4 H(37.5) 63.8 64.7 63.4 64.1 66.6 H(50.0) 65.1 66.9 67.3 65.9 68.1 H(62.5) 66.9 70.2 72.9 69.8 69.7 H(75.0) 68.9 73.8 76.5 75.2 74.5 H(87.5) 72.8 76.9 79.1 78.8 78.4 Hs 76.8 82.4 83.2 81.9 81.7 Hs − H(0.0) 22.8 28.8 31.8 27.7 26.8 Diameter (mm) 31.5 36.3 36.3 36.3 36.3

TABLE II-6 Hardness of Core Comp. Comp. Ex. Ex. Ex. Ex. Ex. II-11 II-12 II-13 II-1 II-2 Composition F G H A A H(0.0) 53.3 56.5 59.2 54.0 54.0 H(12.5) 58.3 59.5 61.4 59.6 59.6 H(25.0) 62.3 61.8 63.0 62.8 62.8 H(37.5) 65.1 62.4 63.8 64.1 64.1 H(50.0) 67.1 66.2 66.5 66.1 66.1 H(62.5) 70.9 73.1 70.6 70.5 70.5 H(75.0) 76.8 74.4 71.4 74.2 74.2 H(87.5) 78.8 76.4 72.8 77.2 77.2 Hs 82.8 79.9 77.6 81.4 81.4 Hs − H(0.0) 29.5 23.4 18.4 27.4 27.4 Diameter (mm) 36.3 36.3 36.3 36.3 36.3

TABLE II-7 Hardness of Core Comp. Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex. II-3 II-4 II-5 II-6 II-7 Composition A A A A A H(0.0) 54.0 54.0 54.0 54.0 54.0 H(12.5) 59.6 59.6 59.7 59.6 59.6 H(25.0) 62.8 62.8 62.9 62.8 62.8 H(37.5) 64.1 64.1 64.2 64.1 64.1 H(50.0) 66.1 66.1 66.6 66.1 66.1 H(62.5) 70.5 70.5 71.4 70.5 70.5 H(75.0) 74.2 74.2 75.4 74.2 74.2 H(87.5) 77.2 77.2 78.7 77.2 77.2 Hs 81.4 81.4 82.1 81.4 81.4 Hs − H(0.0) 27.4 27.4 28.1 27.4 27.4 Diameter (mm) 36.3 36.3 37.9 36.3 36.3

TABLE II-8 Hardness of Core Comp. Comp. Comp. Ex. Ex. Ex. Ex. II-8 II-14 II-9 II-10 Composition I J K A H(0.0) 59.0 57.5 61.9 54.0 H(12.5) 64.1 63.2 63.1 59.6 H(25.0) 66.3 66.1 64.3 62.8 H(37.5) 66.9 68.4 64.1 64.1 H(50.0) 68.2 69.4 66.9 66.1 H(62.5) 70.4 71.2 70.3 70.5 H(75.0) 72.5 74.1 70.4 74.2 H(87.5) 75.3 77.2 68.5 77.2 Hs 81.2 81.1 70.0 81.4 Hs − H(0.0) 22.2 23.6  8.1 27.4 Diameter (mm) 36.3 36.3 36.3 36.3

TABLE II-9 Results of Evaluation Ex. Ex. Ex. Ex. Ex. II-1 II-2 II-3 II-4 II-5 Core Composition A A A A A Acid/salt 10.0 10.0 10.0 10.0 10.0 Comp'n 3.85 3.85 3.85 3.85 3.85 R² 0.99 0.99 0.99 0.99 0.99 Diameter (mm) 36.3 36.3 36.3 36.3 36.3 First cover Composition C4 C4 C2 C2 C4 Thickness T1 (mm) 0.8 0.8 0.8 0.8 0.8 Hardness H1 (JIS-C) 85 85 89 89 85 Second cover Composition C3 C5 C4 C5 C2 Thickness T2 (mm) 0.8 0.8 0.8 0.8 0.8 Hardness H2 (JIS-C) 87 83 85 83 89 Third cover Composition C2 C2 C5 C4 C6 Thickness T3 (mm) 0.8 0.8 0.8 0.8 0.8 Hardness H3 (JIS-C) 89 89 83 85 79 Fourth cover Composition C1 C1 C1 C1 C1 Thickness T4 (mm) 0.8 0.8 0.8 0.8 0.8 Hardness H4 (JIS-C) 92 92 92 92 92 H1 − Hs 3.6 3.6 7.6 7.6 3.6 H4 − H1 7 7 3 3 7 T1 + T2 + T3 + T4 3.2 3.2 3.2 3.2 3.2 (mm) Ball Comp'n 3.21 3.22 3.22 3.22 3.23 Spin (rpm) 3915 4028 3925 3949 4002 Flight distance (m) 146.7 145.8 146.6 146.4 146.0

TABLE II-10 Results of Evaluation Ex. Ex. Ex. Ex. Ex. II-6 II-7 II-8 II-9 II-10 Core Composition A B C D E Acid/salt 10.0 2.5 5.0 10.0 5.0 Comp'n 3.85 3.87 3.83 3.85 3.86 R² 0.97 0.99 0.99 0.99 0.99 Diameter (mm) 31.5 36.3 36.3 36.3 36.3 First cover Composition C4 C4 C4 C4 C4 Thickness T1 (mm) 1.4 0.8 0.8 0.8 0.8 Hardness H1 (JIS-C) 85 85 85 85 85 Second cover Composition C3 C3 C3 C3 C3 Thickness T2 (mm) 1.4 0.8 0.8 0.8 0.8 Hardness H2 (JIS-C) 87 87 87 87 87 Third cover Composition C2 C2 C2 C2 C2 Thickness T3 (mm) 1.4 0.8 0.8 0.8 0.8 Hardness H3 (JIS-C) 89 89 89 89 89 Fourth cover Composition C1 C1 C1 C1 C1 Thickness T4 (mm) 1.4 0.8 0.8 0.8 0.8 Hardness H4 (JIS-C) 92 92 92 92 92 H1 − Hs 8.2 2.6 1.8 3.1 3.3 H4 − H1 7 7 7 7 7 T1 + T2 + T3 + T4 5.6 3.2 3.2 3.2 3.2 (mm) Ball Comp'n 3.19 3.23 3.19 3.21 3.22 Spin (rpm) 4068 3929 3882 3973 3986 Flight distance (m) 145.6 146.6 146.9 146.2 146.1

TABLE II-11 Results of Evaluation Comp. Comp. Ex. Ex. Ex. Ex. Ex. II-11 II-12 II-13 II-1 II-2 Core Composition F G H A A Acid/salt 10.0 20.0 30.0 10.0 10.0 Comp'n 3.84 3.86 3.85 3.85 3.85 R² 0.99 0.97 0.97 0.99 0.99 Diameter (mm) 36.3 36.3 36.3 36.3 36.3 First cover Composition C4 C4 C4 C7 C7 Thickness T1 (mm) 0.8 0.8 0.8 0.8 0.8 Hardness H1 (JIS-C) 85 85 85 76 76 Second cover Composition C3 C3 C3 C4 C6 Thickness T2 (mm) 0.8 0.8 0.8 0.8 0.8 Hardness H2 (JIS-C) 87 87 87 85 79 Third cover Composition C2 C2 C2 C2 C2 Thickness T3 (mm) 0.8 0.8 0.8 0.8 0.8 Hardness H3 (JIS-C) 89 89 89 89 89 Fourth cover Composition C1 C1 C1 C1 C1 Thickness T4 (mm) 0.8 0.8 0.8 0.8 0.8 Hardness H4 (JIS-C) 92 92 92 92 92 H1 − Hs 2.2 5.1 7.4 −5.4 −5.4 H4 − H1 7 7 7 16 16 T1 + T2 + T3 + T4 3.2 3.2 3.2 3.2 3.2 (mm) Ball Comp'n 3.20 3.22 3.21 3.23 3.24 Spin (rpm) 3929 3996 4007 4219 4341 Flight distance (m) 146.6 146.0 145.9 144.2 143.3

TABLE II-12 Results of Evaluation Comp. Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex. II-3 II-4 II-5 II-6 II-7 Core Composition A A A A A Acid/salt 10.0 10.0 10.0 10.0 10.0 Comp'n 3.85 3.85 3.85 3.85 3.85 R² 0.99 0.99 0.99 0.99 0.99 Diameter (mm) 36.3 36.3 37.9 36.3 36.3 First cover Composition C6 C7 C5 C5 C5 Thickness T1 (mm) 0.8 0.8 1.2 1.0 1.0 Hardness H1 (JIS-C) 79 76 83 83 83 Second cover Composition C2 C2 — C4 C6 Thickness T2 (mm) 0.8 0.8 — 1.0 1.0 Hardness H2 (JIS-C) 89 89 — 85 79 Third cover Composition C7 C6 — — — Thickness T3 (mm) 0.8 0.8 — — — Hardness H3 (JIS-C) 76 79 — — — Fourth cover Composition C1 C1 C1 C1 C1 Thickness T4 (mm) 0.8 0.8 1.2 1.2 1.2 Hardness H4 (JIS-C) 92 92 92 92 92 H1 − Hs −2.4 −5.4 0.9 1.6 1.6 H4 − H1 13 16 9 9 9 T1 + T2 + T3 + T4 3.2 3.2 2.4 3.2 3.2 (mm) Ball Comp'n 3.24 3.24 3.22 3.22 3.23 Spin (rpm) 4201 4245 4091 4106 4131 Flight distance (m) 144.4 144.0 145.2 145.2 145.0

TABLE II-13 Results of Evaluation Comp. Comp. Comp. Ex. Ex. Ex. Ex. II-8 II-14 II-9 II-10 Core Composition I J K A Acid/salt 0.0 0.5 40.0 10.0 Comp'n 3.86 3.86 3.86 3.85 R² 0.94 0.97 0.84 0.99 Diameter (mm) 36.3 36.3 36.3 36.3 First cover Composition C4 C4 C4 C3 Thickness T1 (mm) 0.8 0.8 0.8 0.8 Hardness H1 (JIS-C) 85 85 85 87 Second cover Composition C3 C3 C3 C2 Thickness T2 (mm) 0.8 0.8 0.8 0.8 Hardness H2 (JIS-C) 87 87 87 89 Third cover Composition C2 C2 C2 C1 Thickness T3 (mm) 0.8 0.8 0.8 0.8 Hardness H3 (JIS-C) 89 89 89 92 Fourth cover Composition C1 C1 C1 C4 Thickness T4 (mm) 0.8 0.8 0.8 0.8 Hardness H4 (JIS-C) 92 92 92 85 H1 − Hs 3.8 3.9 15.0 5.6 H4 − H1 7 7 7 −2 T1 + T2 + T3 + T4 (mm) 3.2 3.2 3.2 3.2 Ball Comp'n 3.22 3.22 3.22 3.21 Spin (rpm) 4096 4024 4143 4158 Flight distance (m) 145.2 145.8 144.9 144.7

As shown in Tables II-9 to II-13, the golf balls according to Examples have excellent flight performance upon a shot with a middle iron. From the results of evaluation, advantages of the present invention are clear.

Experiment 3 Example III-1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 27 parts by weight of zinc diacrylate (trade name “Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.2 parts by weight of 2-thionaphthol, 10 parts by weight of zinc stearate, and 0.75 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 25 minutes to obtain a core with a diameter of 37.9 mm. The amount of barium sulfate was adjusted such that the weight of a golf ball is 45.4 g.

A resin composition was obtained by kneading 26 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 34 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The core was placed into a mold. The resin composition was injected around the core by injection molding to form an inner cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 45 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 15 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The sphere consisting of the core and the inner cover was placed into a mold. The resin composition was injected around the sphere by injection molding to form a mid cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 5 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 10 parts by weight of another ionomer resin (the aforementioned “Himilan 1555”), 55 parts by weight of still another ionomer resin (the aforementioned “Himilan AM7329”), 30 parts by weight of an ethylene-(meth)acrylic acid copolymer (trade name “NUCREL N1050H”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 3 parts by weight of titanium dioxide, and 0.2 parts by weight of an ultraviolet absorber (trade name “TINUVIN 770”, manufactured by Ciba Japan K.K.) with a twin-screw kneading extruder. The sphere consisting of the core, the inner cover, and the mid cover was placed into a final mold having a large number of pimples on its cavity face. The resin composition was injected around the sphere by injection molding to form an outer cover with a thickness of 0.8 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the outer cover. A clear paint including a two-component curing type polyurethane as a base material was applied to the outer cover to obtain a golf ball of Example III-1 with a diameter of 42.7 mm.

Examples III-2 to III-15 and Comparative Examples III-1 to III-9

Golf balls of Examples III-2 to III-15 and Comparative Examples III-1 to III-9 were obtained in the same manner as Example III-1, except the specifications of the core, the inner cover, the mid cover, and the outer cover were as shown in Tables III-8 to III-12 below. The composition of the core is shown in detail in Tables III-1 to III-3 below. A hardness distribution of the core is shown in Tables III-4 and III-5 below. The compositions of the inner cover, the mid cover, and the outer cover are shown in detail in Tables III-6 and III-7 below. The golf ball according to Comparative Example III-7 does not have a mid cover.

[Hit with Middle Iron (I#5)]

A 5-iron (trade name “XXIO”, manufactured by DUNLOP SPORTS CO. LTD., shaft hardness: R) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the following condition 1, and the spin rate immediately after the hit and the distance from the launch point to the stop point were measured. The average value S1 of spin rates and the average value D1 of flight distances obtained by 10 measurements were calculated. Furthermore, a golf ball was hit under the following condition 2, and the spin rate immediately after the hit and the distance from the launch point to the stop point were measured. The average value S2 of spin rates and the average value D2 of flight distances obtained by 10 measurements were calculated.

Condition 1

-   -   Head speed: 34 m/sec     -   Effective loft angle: 23.5°     -   Hitting point: a point lower than the face center by 5 mm.

Condition 2

-   -   Head speed: 34 m/sec     -   Effective loft angle: 20.5°     -   Hitting point: a point higher than the face center by 5 mm.         The spin rate under the condition 1 is higher than the spin rate         under the condition 2. The flight distance under the condition 1         is smaller than the flight distance under the condition 2. The         average value S1 of the spin rates, the average value D1 of the         flight distances, the average value S2 of the spin rates, the         average value D2 of the flight distances, the difference         (S1−S2), and the difference (D2−D1) are shown in Tables III-8 to         III-12 below.

[Feel at Impact]

Ten golf players hit golf balls with middle irons (I#5) and represented feel at impact of the golf balls as indexes for which 5.0 points are a perfect score. The average value of the indexes is shown as an evaluation result in Tables III-8 to III-12 below. The higher the numerical value is, the better the result is.

TABLE III-1 Composition of Core (parts by weight) A B C D BR-730 100 100 100 100 Sanceler SR 27.0 26.5 27.0 27.5 Zinc oxide 5 5 5 5 Barium sulfate * * * * 2-thionaphthol 0.2 0.2 0.2 0.2 Zinc octoate — — — — Zinc laurate — — — — Zinc myristate — — — — Zinc stearate — 0.5 10.0 20.0 Dicumyl peroxide 0.75 0.75 0.75 0.75 * Appropriate amount

TABLE III-2 Composition of Core (parts by weight) E F G H BR-730 100 100 100 100 Sanceler SR 29.5 31.5 25.5 25.0 Zinc oxide 5 5 5 5 Barium sulfate * * * * 2-thionaphthol 0.2 0.2 0.2 0.2 Zinc octoate — — 2.5 5.0 Zinc laurate — — — — Zinc myristate — — — — Zinc stearate 30.0 40.0 — — Dicumyl peroxide 0.75 0.75 0.75 0.75 * Appropriate amount

TABLE III-3 Composition of Core (parts by weight) I J K BR-730 100 100 100 Sanceler SR 25.0 26.0 25.5 Zinc oxide 5 5 5 Barium sulfate * * * 2-thionaphthol 0.2 0.2 0.2 Zinc octoate — — — Zinc laurate 10.0 — — Zinc myristate — 5.0 10.0 Zinc stearate — — — Dicumyl peroxide 0.75 0.75 0.75 * Appropriate amount

The details of the compounds listed in Tables III-1 to III-3 are as follows.

BR-730: a high-cis polybutadiene manufactured by JSR Corporation (cis-1,4-bond content: 96% by weight, 1,2-vinyl bond content: 1.3% by weight, Mooney viscosity (ML₁₊₄ (100° C.)): 55, molecular weight distribution (Mw/Mn): 3)

Sanceler SR: zinc diacrylate manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD. (a product coated with 10% by weight of stearic acid)

Zinc oxide: trade name “Ginrei R” manufactured by Toho Zinc Co., Ltd.

Barium sulfate: trade name “Barium Sulfate BD” manufactured by Sakai Chemical Industry Co., Ltd.

2-thionaphthol: a product of Tokyo Chemical Industry Co., Ltd.

Zinc octoate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc laurate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc myristate: a product of NOF Corporation

Zinc stearate: a product of Wako Pure Chemical Industries, Ltd.

Dicumyl peroxide: trade name “Percumyl D” manufactured by NOF Corporation

TABLE III-4 Hardness Distribution of Core A B C D E F H(0) 59.0 57.5 54.0 56.5 59.2 61.9 H(12.5) 64.4 63.4 59.7 59.6 61.4 63.1 H(25.0) 67.0 66.5 62.9 61.9 63.1 64.2 H(37.5) 67.4 68.5 64.2 62.5 63.7 64.2 H(50.0) 68.3 69.6 66.6 66.3 66.5 66.8 H(62.5) 70.2 70.8 71.4 73.4 70.6 70.3 H(75.0) 73.6 74.3 75.4 75.0 71.8 70.4 H(87.5) 78.2 78.1 78.7 77.8 72.6 68.4 Hs 82.3 82.2 82.1 80.9 78.4 70.2 Hs − H(0) 23.3 24.7 28.1 24.4 19.2 8.3 Diameter (mm) 37.9 37.9 37.9 37.9 37.9 37.9 Comp'n 3.86 3.86 3.85 3.86 3.85 3.86

TABLE III-5 Hardness Distribution of Core G H I J K H(0) 53.6 51.4 54.2 54.9 53.3 H(12.5) 58.2 57.5 58.1 58.9 58.3 H(25.0) 61.6 61.1 62.0 63.5 62.5 H(37.5) 64.9 63.4 64.1 66.8 65.3 H(50.0) 67.1 67.5 66.0 68.2 67.2 H(62.5) 70.6 73.4 70.6 69.7 71.5 H(75.0) 74.8 77.3 76.1 76.2 76.9 H(87.5) 79.9 81.2 79.7 79.6 80.4 Hs 83.6 84.2 82.8 82.6 83.7 Hs − H(0) 30.0 32.8 28.6 27.7 30.4 Diameter (mm) 37.9 37.9 37.9 37.9 37.9 Comp'n 3.87 3.83 3.85 3.86 3.84

TABLE III-6 Composition of Cover (parts by weight) C1 C2 C3 C4 Himilan AM7337 5 45 40 24 Himilan 1555 10 — — — Himilan AM7329 55 40 40 50 NUCREL N1050H 30 — — — Rabalon T3221C — 15 20 26 Titanium dioxide 3  6  6  6 (A220) TINUVIN 770 0.2 — — — Hardness (JIS-C) 92 87 85 83 Hardness (Shore D) 61 56 54 52

TABLE III-7 Composition of Cover (parts by weight) C5 C6 C7 C8 Himilan AM7337 30 26 30 26 Himilan 1555 — — — Himilan AM7329 40 40 30 30 NUCREL N1050H — — — Rabalon T3221C 30 34 40 44 Titanium dioxide  6  6  6  6 (A220) TINUVIN 770 — — — Hardness (JIS-C) 79 76 71 68 Hardness (Shore D) 48 45 40 37

TABLE III-8 Results of Evaluation Ex. Ex. Ex. Ex. Ex. III-1 III-2 III-3 III-4 III-5 Core Composition C C C C C Acid and/or salt (PHR) 10.0 10.0 10.0 10.0 10.0 R² 0.99 0.99 0.99 0.99 0.99 Hs − H(0) 28.1 28.1 28.1 28.1 28.1 Diameter (mm) 37.9 37.9 37.9 37.9 37.9 Comp'n 3.85 3.85 3.85 3.85 3.85 Inner cover Composition C6 C6 C6 C5 C7 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hi (JIS-C) 76.0 76.0 76.0 79.0 71.0 Mid cover Composition C2 C4 C7 C4 C6 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hm (JIS-C) 87.0 83.0 71.0 83.0 76.0 Outer cover Composition C1 C1 C1 C1 C1 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho (JIS-C) 92.0 92.0 92.0 92.0 92.0 Ball Ho − Hi 16.0 16.0 16.0 13.0 21.0 Hs − Hi 6.1 6.1 6.1 3.1 11.1 Cover thickness (mm) 2.4 2.4 2.4 2.4 2.4 Comp'n 3.25 3.27 3.29 3.25 3.28 Spin S1 (rpm) 4524 4588 4703 4516 4751 Flight distance D1 (m) 140.9 140.4 139.5 141.0 139.1 Spin S2 (rpm) 3518 3649 3800 3553 3852 Flight distance D2 (m) 149.0 147.9 146.7 148.7 146.3 S1 − S2 (rpm) 1006 939 903 963 899 D1 − D2 (m) 8.0 7.5 7.2 7.7 7.2 Feel at impact 3.5 3.8 4.5 3.6 4.3

TABLE III-9 Results of Evaluation Ex. Ex. Ex. Ex. Ex. III-6 III-7 III-8 III-9 III-10 Core Composition G H I J K Acid and/or salt (PHR) 2.5 5.0 10.0 5.0 10.0 R² 0.99 0.99 0.99 0.98 0.99 Hs − H(0) 30.0 32.8 28.6 27.7 30.4 Diameter (mm) 37.9 37.9 37.9 37.9 37.9 Comp'n 3.87 3.83 3.85 3.86 3.84 Inner cover Composition C6 C6 C6 C6 C6 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hi (JIS-C) 76.0 76.0 76.0 76.0 76.0 Mid cover Composition C4 C4 C4 C4 C4 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hm (JIS-C) 83.0 83.0 83.0 83.0 83.0 Outer cover Composition C1 C1 C1 C1 C1 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho (JIS-C) 92.0 92.0 92.0 92.0 92.0 Ball Ho − Hi 16.0 16.0 16.0 16.0 16.0 Hs − Hi 7.6 8.2 6.8 6.6 7.7 Cover thickness (mm) 2.4 2.4 2.4 2.4 2.4 Comp'n 3.29 3.25 3.27 3.28 3.26 Spin S1 (rpm) 4548 4513 4579 4596 4549 Flight distance D1 (m) 140.7 141.0 140.5 140.3 140.7 Spin S2 (rpm) 3576 3527 3627 3653 3589 Flight distance D2 (m) 148.5 148.9 148.1 147.9 148.4 S1 − S2 (rpm) 972 986 952 943 960 D1 − D2 (m) 7.8 7.9 7.6 7.5 7.7 Feel at impact 3.8 3.9 3.7 3.5 3.7

TABLE III-10 Results of Evaluation Ex. Ex. Ex. Ex. Ex. III-11 III-12 III-13 III-14 III-15 Core Composition D E C B C Acid and/or salt (PHR) 20.0 30.0 10.0 0.5 10.0 R² 0.97 0.96 0.99 0.96 0.99 Hs − H(0) 24.4 19.2 27.6 24.7 28.1 Diameter (mm) 37.9 37.9 37.1 37.9 37.9 Comp'n 3.86 3.85 3.85 3.86 3.85 Inner cover Composition C6 C6 C5 C6 C6 Thickness (mm) 0.8 0.8 1.0 0.8 0.8 Hardness Hi (JIS-C) 76.0 76.0 79.0 76.0 76.0 Mid cover Composition C4 C4 C4 C4 C8 Thickness (mm) 0.8 0.8 1.0 0.8 0.8 Hardness Hm (JIS-C) 83.0 83.0 83.0 83.0 68.0 Outer cover Composition C1 C1 C1 C1 C1 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho (JIS-C) 92.0 92.0 92.0 92.0 92.0 Ball Ho − Hi 16.0 16.0 13.0 16.0 16.0 Hs − Hi 4.9 2.4 2.6 6.2 6.1 Cover thickness (mm) 2.4 2.4 2.8 2.4 2.4 Comp'n 3.28 3.27 3.25 3.28 3.32 Spin S1 (rpm) 4632 4646 4584 4612 4788 Flight distance D1 (m) 140.0 139.9 140.4 140.2 138.8 Spin S2 (rpm) 3654 3707 3591 3634 3871 Flight distance D2 (m) 147.9 147.4 148.4 148.0 146.1 S1 − S2 (rpm) 978 939 993 980 917 D1 − D2 (m) 7.8 7.5 7.9 7.8 7.3 Feel at impact 3.4 3.4 3.4 3.3 4.7

TABLE III-11 Results of Evaluation Comp. Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex. III-1 III-2 III-3 III-4 III-5 Core Composition F C C C C Acid and/or salt (PHR) 40.0 10.0 10.0 10.0 10.0 R² 0.84 0.99 0.99 0.99 0.99 Hs − H(0) 8.3 28.1 28.1 28.1 28.1 Diameter (mm) 37.9 37.9 37.9 37.9 37.9 Comp'n 3.86 3.85 3.85 3.85 3.85 Inner cover Composition C6 C2 C2 C4 C3 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hi (JIS-C) 76.0 87.0 87.0 83.0 85.0 Mid cover Composition C4 C6 C4 C2 C5 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hm (JIS-C) 83.0 76.0 83.0 87.0 79.0 Outer cover Composition C1 C1 C1 C1 C1 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho (JIS-C) 92.0 92.0 92.0 92.0 92.0 Ball Ho − Hi 16.0 5.0 5.0 9.0 7.0 Hs − Hi −5.8 −4.9 −4.9 −0.9 −2.9 Cover thickness (mm) 2.4 2.4 2.4 2.4 2.4 Comp'n 3.28 3.25 3.23 3.23 3.25 Spin S1 (rpm) 4811 4465 4415 4446 4455 Flight distance D1 (m) 138.4 141.4 141.8 141.5 141.5 Spin S2 (rpm) 3783 3292 3192 3298 3384 Flight distance D2 (m) 146.7 150.8 151.6 150.7 150.0 S1 − S2 (rpm) 1032 1173 1223 1148 1071 D1 − D2 (m) 8.3 9.4 9.8 9.2 8.6 Feel at impact 3.2 2.3 1.7 1.9 2.6

TABLE III-12 Results of Evaluation Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. III-6 III-7 III-8 III-9 Core Composition C C A C Acid and/or salt (PHR) 10.0 10.0 10.0 10.0 R² 0.99 0.99 0.94 0.99 Hs − H(0) 28.1 28.1 23.3 28.1 Diameter (mm) 37.9 37.9 37.9 37.9 Comp'n 3.85 3.85 3.86 3.85 Inner cover Composition C3 C5 C6 C6 Thickness (mm) 0.8 1.2 0.8 0.8 Hardness Hi (JIS-C) 85.0 79.0 76.0 76.0 Mid cover Composition C7 — C4 C4 Thickness (mm) 0.8 — 0.8 0.8 Hardness Hm (JIS-C) 71.0 — 83.0 83.0 Outer cover Composition C1 C1 C1 C7 Thickness (mm) 0.8 1.2 0.8 0.8 Hardness Ho (JIS-C) 92.0 92.0 92.0 71.0 Ball Ho − Hi 7.0 13.0 16.0 −5.0 Hs − Hi −2.9 3.1 6.3 6.1 Cover thickness (mm) 2.4 2.4 2.4 2.4 Comp'n 3.27 3.24 3.28 3.32 Spin S1 (rpm) 4516 4483 4682 4846 Flight distance D1 (m) 141.0 141.2 139.6 138.5 Spin S2 (rpm) 3450 3412 3624 3798 Flight distance D2 (m) 149.5 149.8 148.1 147.0 S1 − S2 (rpm) 1066 1071 1058 1048 D1 − D2 (m) 8.5 8.6 8.5 8.5 Feel at impact 3.1 2.1 3.1 3.2

As shown in Tables III-8 to III-12, the golf balls according to Examples have excellent feel at impact and excellent flight performance upon a shot with a middle iron. From the results of evaluation, advantages of the present invention are clear.

Experiment 4 Example IV-1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 27 parts by weight of zinc diacrylate (trade name “Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.2 parts by weight of 2-thionaphthol, 10 parts by weight of zinc stearate, and 0.75 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 25 minutes to obtain a core with a diameter of 36.3 mm. The amount of barium sulfate was adjusted such that the weight of a golf ball is 45.4 g.

A resin composition was obtained by kneading 26 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 34 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The core was placed into a mold. The resin composition was injected around the core by injection molding to form an inner cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 24 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 50 parts by weight of another ionomer resin (the aforementioned“Himilan AM7329”), 26 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The sphere consisting of the core and the inner cover was placed into a mold. The resin composition was injected around the sphere by injection molding to form a first mid cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 45 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 15 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The sphere consisting of the core, the inner cover, and the first mid cover was placed into a mold. The resin composition was injected around the sphere by injection molding to form a second mid cover with a thickness of 0.8 mm.

A resin composition was obtained by kneading 5 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 10 parts by weight of another ionomer resin (the aforementioned “Himilan 1555”), 55 parts by weight of still another ionomer resin (the aforementioned “Himilan AM7329”), 30 parts by weight of an ethylene-(meth)acrylic acid copolymer (trade name “NUCREL N1050H”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 3 parts by weight of titanium dioxide, and 0.2 parts by weight of an ultraviolet absorber (trade name “TINUVIN 770”, manufactured by Ciba Japan K.K.) with a twin-screw kneading extruder. The sphere consisting of the core, the inner cover, the first mid cover, and the second mid cover was placed into a final mold having a large number of pimples on its cavity face. The resin composition was injected around the sphere by injection molding to form an outer cover with a thickness of 0.8 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the outer cover. A clear paint including a two-component curing type polyurethane as a base material was applied to the outer cover to obtain a golf ball of Example IV-1 with a diameter of 42.7 mm.

Examples IV-2 to IV-14 and Comparative Examples IV-1 to IV-11

Golf balls of Examples IV-2 to IV-14 and Comparative Examples IV-1 to IV-11 were obtained in the same manner as Example IV-1, except the specifications of the core, the inner cover, the first mid cover, the second mid cover, and the outer cover were as shown in Tables IV-11 to IV-15 below. The composition of the core is shown in detail in Tables IV-1 to IV-3 below. The compositions of the inner cover, the first mid cover, the second mid cover, and the outer cover are shown in detail in Tables IV-4 and IV-5 below. A hardness distribution of the core is shown in Tables IV-6 to IV-10 below. The golf ball according to Comparative Example IV-5 does not have a first mid cover and a second mid cover. Each of the golf balls according to Comparative Examples IV-6 and IV-7 does not have a second mid cover.

[Hit with Middle Iron (145)]

A 5-iron (trade name “XXIO”, manufactured by DUNLOP SPORTS CO. LTD., shaft hardness: R) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the following condition 1, and the spin rate immediately after the hit and the distance from the launch point to the stop point were measured. The average value S1 of spin rates and the average value D1 of flight distances obtained by 10 measurements were calculated. In addition, a golf ball was hit under the following condition 2, and the spin rate immediately after the hit and the distance from the launch point to the stop point were measured. The average value S2 of spin rates and the average value D2 of flight distances obtained by 10 measurements were calculated. Furthermore, a golf ball was hit under the following condition 3, and the spin rate immediately after the hit and the distance from the launch point to the stop point were measured. The average value S3 of spin rates and the average value D3 of flight distances obtained by 10 measurements were calculated. The conditions 1 and 2 are conditions for confirming variations in spin rate and flight distance. The condition 3 is a normal condition for confirming a spin rate and a flight distance.

Condition 1

-   -   Head speed: 34 m/sec     -   Effective loft angle: 23.5°     -   Hitting point: a point lower than the face center by 5 mm.

Condition 2

-   -   Head speed: 34 m/sec     -   Effective loft angle: 20.5°     -   Hitting point: a point higher than the face center by 5 mm.

Condition 3

-   -   Head speed: 34 m/sec     -   Effective loft angle: 23.0°     -   Hitting point: the face center         The spin rate under the condition 1 is higher than the spin rate         under the condition 2. The flight distance under the condition 1         is smaller than the flight distance under the condition 2. The         average value S1 of the spin rates, the average value D1 of the         flight distances, the average value S2 of the spin rates, the         average value D2 of the flight distances, the difference         (S1−S2), the difference (D2−D1), the average value S3 of the         spin rates, and the average value D3 of the flight distances are         shown in Tables IV-11 to IV-15 below.

[Feel at Impact]

Ten golf players hit golf balls with middle irons (I#5) and represented feel at impact of the golf balls as indexes for which 5.0 points are a perfect score. The average value of the indexes is shown as an evaluation result in Tables IV-11 to IV-15 below. The higher the numerical value is, the better the result is.

TABLE IV-1 Composition of Core (parts by weight) A B C D BR-730 100 100 100 100 Sanceler SR 27.0 25.5 25.0 25.0 Zinc oxide 5 5 5 5 Barium sulfate * * * * 2-thionaphthol 0.2 0.2 0.2 0.2 Zinc octoate — 2.5 5.0 — Zinc laurate — — — 10.0 Zinc myristate — — — — Zinc stearate 10.0 — — — Dicumyl peroxide 0.75 0.75 0.75 0.75 * Appropriate amount

TABLE IV-2 Composition of Core (parts by weight) E F G H BR-730 100 100 100 100 Sanceler SR 26.0 25.5 27.5 29.5 Zinc oxide 5 5 5 5 Barium sulfate * * * * 2-thionaphthol 0.2 0.2 0.2 0.2 Zinc octoate — — — — Zinc laurate — — — — Zinc myristate 5.0 10.0 — — Zinc stearate — — 20.0 30.0 Dicumyl peroxide 0.75 0.75 0.75 0.75 * Appropriate amount

TABLE IV-3 Composition of Core (parts by weight) I J K BR-730 100 100 100 Sanceler SR 27.0 26.5 31.5 Zinc oxide 5 5 5 Barium sulfate * * * 2-thionaphthol 0.2 0.2 0.2 Zinc octoate — — — Zinc laurate — — — Zinc myristate — — — Zinc stearate — 0.5 40.0 Dicumyl peroxide 0.75 0.75 0.75 * Appropriate amount

The details of the compounds listed in Tables IV-1 to IV-3 are as follows.

BR-730: a high-cis polybutadiene manufactured by JSR Corporation (cis-1,4-bond content: 96% by weight, 1,2-vinyl bond content: 1.3% by weight, Mooney viscosity (ML₁₊₄ (100° C.)): 55, molecular weight distribution (Mw/Mn): 3)

Sanceler SR: zinc diacrylate manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD. (a product coated with 10% by weight of stearic acid)

Zinc oxide: trade name “Ginrei R” manufactured by Toho Zinc Co., Ltd.

Barium sulfate: trade name “Barium Sulfate BD” manufactured by Sakai Chemical Industry Co., Ltd.

2-thionaphthol: a product of Tokyo Chemical Industry Co., Ltd.

Zinc octoate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc laurate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc myristate: a product of NOF Corporation

Zinc stearate: a product of Wako Pure Chemical Industries, Ltd.

Dicumyl peroxide: trade name “Percumyl D” manufactured by NOF Corporation

TABLE IV-4 Composition of Cover (parts by weight) C1 C2 C3 C4 Himilan AM7337 5 45 40 24 Himilan 1555 10 — — — Himilan AM7329 55 40 40 50 NUCREL N1050H 30 — — — Rabalon T3221C — 15 20 26 Titanium dioxide (A220) 3  6  6  6 TINUVIN 770 0.2 — — — Hardness (JIS-C) 92 87 85 83 Hardness (Shore D) 61 56 54 52

TABLE IV-5 Composition of Cover (parts by weight) C5 C6 C7 Himilan AM7337 30 26 30 Himilan 1555 — — — Himilan AM7329 40 40 30 NUCREL N1050H — — — Rabalon T3221C 30 34 40 Titanium dioxide (A220)  6  6  6 TINUVIN 770 — — — Hardness (JIS-C) 79 76 71 Hardness (Shore D) 48 45 40

TABLE IV-6 Hardness Distribution of Core Ex. Ex. Ex. Ex. Ex. IV-1 IV-2 IV-3 IV-4 IV-5 H(0) 54.0 54.0 54.0 54.0 54.0 H(12.5) 59.6 59.6 59.6 59.6 59.6 H(25.0) 62.8 62.8 62.8 62.8 62.8 H(37.5) 64.1 64.1 64.1 64.1 64.1 H(50.0) 66.1 66.1 66.1 66.1 66.1 H(62.5) 70.5 70.5 70.5 70.5 70.5 H(75.0) 74.2 74.2 74.2 74.2 74.2 H(87.5) 77.2 77.2 77.2 77.2 77.2 Hs 81.4 81.4 81.4 81.4 81.4

TABLE IV-7 Hardness Distribution of Core Ex. Ex. Ex. Ex. Ex. IV-6 IV-7 IV-8 IV-9 IV-10 H(0) 54.0 53.6 51.4 54.2 54.9 H(12.5) 58.8 58.1 57.5 58.1 58.9 H(25.0) 61.4 61.3 60.9 61.9 63.4 H(37.5) 63.8 64.7 63.4 64.1 66.6 H(50.0) 65.1 66.9 67.3 65.9 68.1 H(62.5) 66.9 70.2 72.9 69.8 69.7 H(75.0) 68.9 73.8 76.5 75.2 74.5 H(87.5) 72.8 76.9 79.1 78.8 78.4 Hs 76.8 82.4 83.2 81.9 81.7

TABLE IV-8 Hardness Distribution of Core Comp. Comp. Ex. Ex. Ex. Ex. Ex. IV-11 IV-12 IV-13 IV-1 IV-2 H(0) 53.3 56.5 59.2 54.0 54.0 H(12.5) 58.3 59.5 61.4 59.6 59.6 H(25.0) 62.3 61.8 63.0 62.8 62.8 H(37.5) 65.1 62.4 63.8 64.1 64.1 H(50.0) 67.1 66.2 66.5 66.1 66.1 H(62.5) 70.9 73.1 70.6 70.5 70.5 H(75.0) 76.8 74.4 71.4 74.2 74.2 H(87.5) 78.8 76.4 72.8 77.2 77.2 Hs 82.8 79.9 77.6 81.4 81.4

TABLE IV-9 Hardness Distribution of Core Comp. Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex. IV-3 IV-4 IV-5 IV-6 IV-7 H(0) 54.0 54.0 54.0 54.0 54.0 H(12.5) 59.6 59.6 59.7 59.6 59.6 H(25.0) 62.8 62.8 62.9 62.8 62.8 H(37.5) 64.1 64.1 64.2 64.1 64.1 H(50.0) 66.1 66.1 66.6 66.1 66.1 H(62.5) 70.5 70.5 71.4 70.5 70.5 H(75.0) 74.2 74.2 75.4 74.2 74.2 H(87.5) 77.2 77.2 78.7 77.2 77.2 Hs 81.4 81.4 82.1 81.4 81.4

TABLE IV-10 Hardness Distribution of Core Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex. IV-8 IV-14 IV-9 IV-10 IV-11 H(0) 59.0 57.5 61.9 54.0 54.0 H(12.5) 64.1 63.2 63.1 59.6 59.6 H(25.0) 66.3 66.1 64.3 62.8 62.8 H(37.5) 66.9 68.4 64.1 64.1 64.1 H(50.0) 68.2 69.4 66.9 66.1 66.1 H(62.5) 70.4 71.2 70.3 70.5 70.5 H(75.0) 72.5 74.1 70.4 74.2 74.2 H(87.5) 75.3 77.2 68.5 77.2 77.2 Hs 81.2 81.1 70.0 81.4 81.4

TABLE IV-11 Results of Evaluation Ex. Ex. Ex. Ex. Ex. IV-1 IV-2 IV-3 IV-4 IV-5 Core Composition A A A A A Acid and/or salt 10.0 10.0 10.0 10.0 10.0 (PHR) R² 0.99 0.99 0.99 0.99 0.99 Hs − H(0) 27.4 27.4 27.4 27.4 27.4 Diameter (mm) 36.3 36.3 36.3 36.3 36.3 Comp'n 3.85 3.85 3.85 3.85 3.85 Inner Composition C6 C6 C5 C5 C5 cover Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hi 76.0 76.0 79.0 79.0 79.0 (JIS-C) 1st Composition C4 C4 C6 C6 C7 mid Thickness (mm) 0.8 0.8 0.8 0.8 0.8 cover Hardness Hm1 83.0 83.0 76.0 76.0 71.0 (JIS-C) 2nd Composition C2 C7 C2 C7 C6 mid Thickness (mm) 0.8 0.8 0.8 0.8 0.8 cover Hardness Hm2 87.0 71.0 87.0 71.0 76.0 (JIS-C) Outer Composition C1 C1 C1 C1 C1 cover Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho 92.0 92.0 92.0 92.0 92.0 (JIS-C) Ball Ho − Hi 16.0 16.0 13.0 13.0 13.0 Hs − Hi 5.4 5.4 2.4 2.4 2.4 Cover thickness 3.2 3.2 3.2 3.2 3.2 (mm) Comp'n 3.25 3.27 3.26 3.28 3.28 Spin S1 (rpm) 4684 4852 4689 4902 4912 Flight distance D1 (m) 140.4 139.1 140.4 138.7 138.6 Spin S2 (rpm) 3687 3913 3786 3939 4013 Flight distance D2 (m) 148.4 146.6 147.6 146.4 145.8 S1 − S2 (rpm) 997 939 903 963 899 D1 − D2 (m) 8.0 7.5 7.2 7.7 7.2 Spin S3 (rpm) 4186 4376 4227 4417 4440 Flight distance D3 (m) 144.4 142.9 144.1 142.6 142.4 Feel at impact 3.6 4.2 4.1 4.4 4.7

TABLE IV-12 Results of Evaluation Ex. Ex. Ex. Ex. Ex. IV-6 IV-7 IV-8 IV-9 IV-10 Core Composition A B C D E Acid and/or salt 10.0 2.5 5.0 10.0 5.0 (PHR) R² 0.97 0.99 0.99 0.99 0.99 Hs − H(0) 22.8 30.0 32.8 28.6 27.7 Diameter (mm) 31.5 36.3 36.3 36.3 36.3 Comp'n 3.85 3.87 3.83 3.85 3.86 Inner Composition C6 C6 C6 C6 C6 cover Thickness (mm) 1.4 0.8 0.8 0.8 0.8 Hardness Hi 76.0 76.0 76.0 76.0 76.0 (JIS-C) 1st Composition C4 C4 C4 C4 C4 mid Thickness (mm) 1.4 0.8 0.8 0.8 0.8 cover Hardness Hm1 83.0 83.0 83.0 83.0 83.0 (JIS-C) 2nd Composition C2 C2 C2 C2 C2 mid Thickness (mm) 1.4 0.8 0.8 0.8 0.8 cover Hardness Hm2 87.0 87.0 87.0 87.0 87.0 (JIS-C) Outer Composition C1 C1 C1 C1 C1 cover Thickness (mm) 1.4 0.8 0.8 0.8 0.8 Hardness Ho 92.0 92.0 92.0 92.0 92.0 (JIS-C) Ball Ho − Hi 16.0 16.0 16.0 16.0 16.0 Hs − Hi 0.8 6.4 7.2 5.9 5.7 Cover thickness 5.6 3.2 3.2 3.2 3.2 (mm) Comp'n 3.18 3.27 3.23 3.25 3.26 Spin S1 (rpm) 4824 4708 4862 4780 4984 Flight distance D1 (m) 139.3 140.2 139.0 139.7 138.0 Spin S2 (rpm) 3873 3748 3920 3900 4080 Flight distance D2 (m) 146.9 147.9 146.5 146.7 145.3 S1 − S2 (rpm) 951 960 942 880 904 D1 − D2 (m) 7.6 7.7 7.5 7.0 7.2 Spin S3 (rpm) 4341 4219 4361 4308 4515 Flight distance D3 (m) 143.2 144.1 143.0 143.4 141.8 Feel at impact 3.4 3.7 3.8 3.7 3.4

TABLE IV-13 Results of Evaluation Comp. Comp. Ex. Ex. Ex. Ex. Ex. IV-11 IV-12 IV-13 IV-1 IV-2 Core Composition F G H A A Acid and/or salt 10.0 20.0 30.0 10.0 10.0 (PHR) R² 0.99 0.97 0.97 0.99 0.99 Hs − H(0) 30.4 24.4 19.2 27.4 27.4 Diameter (mm) 36.3 36.3 36.3 36.3 36.3 Comp'n 3.84 3.86 3.85 3.85 3.85 Inner Composition C6 C6 C6 C2 C2 cover Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Hi 76.0 76.0 76.0 87.0 87.0 (JIS-C) 1st Composition C4 C4 C4 C4 C6 mid Thickness (mm) 0.8 0.8 0.8 0.8 0.8 cover Hardness Hm1 83.0 83.0 83.0 83.0 76.0 (JIS-C) 2nd Composition C2 C2 C2 C6 C4 mid Thickness (mm) 0.8 0.8 0.8 0.8 0.8 cover Hardness Hm2 87.0 87.0 87.0 76.0 83.0 (JIS-C) Outer Composition C1 C1 C1 C1 C1 cover Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho 92.0 92.0 92.0 92.0 92.0 (JIS-C) Ball Ho − Hi 16.0 16.0 16.0 5.0 5.0 Hs − Hi 6.8 3.9 1.6 −5.6 −5.6 Cover thickness 3.2 3.2 3.2 3.2 3.2 (mm) Comp'n 3.24 3.26 3.25 3.25 3.25 Spin S1 (rpm) 4961 4830 4988 4605 4687 Flight distance D1 (m) 138.2 139.3 138.0 141.1 140.4 Spin S2 (rpm) 4088 3895 4122 3432 3464 Flight distance D2 (m) 145.2 146.7 144.9 150.4 150.2 S1 − S2 (rpm) 873 935 866 1173 1223 D1 − D2 (m) 7.0 7.5 6.9 9.4 9.8 Spin S3 (rpm) 4477 4340 4512 4014 4060 Flight distance D3 (m) 142.1 143.2 141.8 145.8 145.4 Feel at impact 3.7 3.3 3.2 2.6 2.8

TABLE IV-14 Results of Evaluation Comp. Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex. IV-3 IV-4 IV-5 IV-6 IV-7 Core Composition A A A A A Acid and/or salt 10.0 10.0 10.0 10.0 10.0 (PHR) R² 0.99 0.99 0.99 0.99 0.99 Hs − H(0) 27.4 27.4 28.1 27.4 27.4 Diameter (mm) 36.3 36.3 37.9 36.3 36.3 Comp'n 3.85 3.85 3.85 3.85 3.85 Inner Composition C4 C4 C5 C5 C5 cover Thickness (mm) 0.8 0.8 1.2 1.0 1.0 Hardness Hi 83.0 83.0 79.0 79.0 79.0 (JIS-C) 1st Composition C3 C2 — C3 C7 mid Thickness (mm) 0.8 0.8 — 1.0 1.0 cover Hardness Hm1 85.0 87.0 — 85.0 71.0 (JIS-C) 2nd Composition C2 C3 — — — mid Thickness (mm) 0.8 0.8 — — — cover Hardness Hm2 87.0 85.0 — — — (JIS-C) Outer Composition C1 C1 C1 C1 C1 cover Thickness (mm) 0.8 0.8 1.2 1.2 1.2 Hardness Ho 92.0 92.0 92.0 92.0 92.0 (JIS-C) Ball Ho − Hi 9.0 9.0 13.0 13.0 13.0 Hs − Hi −1.6 −1.6 3.1 2.4 2.4 Cover thickness 3.2 3.2 2.4 3.2 3.2 (mm) Comp'n 3.22 3.22 3.26 3.24 3.27 Spin S1 (rpm) 4545 4483 4648 4789 4800 Flight distance D1 (m) 141.5 142.0 140.7 139.6 139.5 Spin S2 (rpm) 3397 3412 3583 3657 3742 Flight distance D2 (m) 150.7 150.6 149.2 148.6 148.0 S1 − S2 (rpm) 1148 1071 1065 1132 1058 D1 − D2 (m) 9.2 8.6 8.5 9.1 8.5 Spin S3 (rpm) 3963 3940 4112 4218 4259 Flight distance D3 (m) 146.2 146.4 145.0 144.2 143.8 Feel at impact 2.1 1.7 3.1 2.9 3.1

TABLE IV-15 Results of Evaluation Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex. IV-8 IV-14 IV-9 IV-10 IV-11 Core Composition I J K A A Acid and/or salt 0.0 0.5 40.0 10.0 10.0 (PHR) R² 0.94 0.97 0.84 0.99 0.99 Hs − H(0) 22.2 23.6 8.1 27.4 27.4 Diameter (mm) 36.3 36.3 36.3 36.3 36.3 Comp'n 3.86 3.86 3.86 3.85 3.85 Inner Composition C6 C6 C6 C3 C5 cover Thickness (mm) 0.8 0.8 0.8 1.0 0.8 Hardness Hi 76.0 76.0 76.0 85.0 79.0 (JIS-C) 1st Composition C4 C4 C4 C4 C4 mid Thickness (mm) 0.8 0.8 0.8 0.8 0.8 cover Hardness Hm1 83.0 83.0 83.0 83.0 83.0 (JIS-C) 2nd Composition C2 C2 C2 C2 C2 mid Thickness (mm) 0.8 0.8 0.8 0.8 0.8 cover Hardness Hm2 87.0 87.0 87.0 87.0 87.0 (JIS-C) Outer Composition C1 C1 C1 C1 C6 cover Thickness (mm) 0.8 0.8 0.8 0.8 0.8 Hardness Ho 92.0 92.0 92.0 92.0 76.0 (JIS-C) Ball Ho − Hi 16.0 16.0 16.0 7.0 −3.0 Hs − Hi 5.2 5.1 −6.0 −3.6 2.4 Cover thickness 3.2 3.2 3.2 3.2 3.2 (mm) Comp'n 3.26 3.26 3.26 3.22 3.27 Spin S1 (rpm) 4846 4812 5009 4554 4659 Flight distance D1 (m) 139.1 139.4 137.8 141.5 140.3 Spin S2 (rpm) 3781 3803 3976 3395 3615 Flight distance D2 (m) 147.7 147.5 146.1 150.7 149.1 S1 − S2 (rpm) 1065 1009 1033 1159 1044 D1 − D2 (m) 8.5 8.0 8.3 9.3 8.4 Spin S3 (rpm) 4302 4278 4420 3943 4143 Flight distance D3 (m) 143.5 143.7 142.5 146.4 144.4 Feel at impact 3.0 3.2 2.9 2.1 3.1

As shown in Tables IV-11 to IV-15, the golf balls according to Examples have excellent feel at impact and excellent flight performance upon a shot with a middle iron. From the results of evaluation, advantages of the present invention are clear.

The golf ball according to the present invention can be used for playing golf on golf courses and practicing at driving ranges. The above descriptions are merely for illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

What is claimed is:
 1. A golf ball comprising a core, an inner cover positioned outside the core, a mid cover positioned outside the inner cover, and an outer cover positioned outside the mid cover, wherein when distances (%) from a central point of the core to nine points and JIS-C hardnesses at the nine points, which nine points are obtained by dividing a region from the central point of the core to a surface of the core at intervals of 12.5% of a radius of the core, are plotted in a graph, R² of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95, a JIS-C hardness Hi of the inner cover is greater than a JIS-C hardness Hs at the surface of the core, and a JIS-C hardness Ho of the outer cover is greater than the hardness Hi.
 2. The golf ball according to claim 1, wherein a difference (Hi−Hs) between the hardness Hi and the hardness Hs is equal to or greater than
 1. 3. The golf ball according to claim 1, wherein a difference (Ho−Hi) between the hardness Ho and the hardness Hi is equal to or greater than 5 but equal to or less than
 30. 4. The golf ball according to claim 1, wherein a difference (Hs−H(0)) between the hardness Hs and a JIS-C hardness H(0) at the central point of the core is equal to or greater than
 15. 5. The golf ball according to claim 1, wherein a thickness Ti of the inner cover is equal to or less than 1.2 mm, a thickness Tm of the mid cover is equal to or less than 1.2 mm, and a thickness To of the outer cover is equal to or less than 1.2 mm.
 6. The golf ball according to claim 1, wherein a sum (Ti+Tm+To) of a thickness Ti of the inner cover, a thickness Tm of the mid cover, and a thickness To of the outer cover is equal to or less than 2.5 mm.
 7. The golf ball according to claim 1, wherein the hardness Ho is greater than a JIS-C hardness Hm of the mid cover, and a difference (Ho−Hm) between the hardness Ho and the hardness Hm is equal to or greater than
 2. 8. The golf ball according to claim 1, wherein the core is formed by a rubber composition being crosslinked, the rubber composition includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, and the co-crosslinking agent (b) is: (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and/or (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.
 9. The golf ball according to claim 8, wherein an amount of the acid and/or the salt (d) is equal to or greater than 0.5 parts by weight but equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber (a).
 10. The golf ball according to claim 8, wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1).
 11. The golf ball according to claim 10, wherein the carboxylic acid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof.
 12. The golf ball according to claim 10, wherein a carbon number of a carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is equal to or greater than 1 but equal to or less than
 30. 13. The golf ball according to claim 10, wherein the carboxylic acid and/or the salt thereof (d1) is a zinc salt of a carboxylic acid.
 14. The golf ball according to claim 13, wherein the zinc salt of the carboxylic acid is one or more members selected from the group consisting of zinc octoate, zinc laurate, zinc myristate, and zinc stearate.
 15. The golf ball according to claim 8, wherein the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a).
 16. The golf ball according to claim 8, wherein the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a).
 17. The golf ball according to claim 8, wherein the rubber composition further includes an organic sulfur compound (e).
 18. The golf ball according to claim 17, wherein the organic sulfur compound (e) is at least one member selected from the group consisting of thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.
 19. The golf ball according to claim 17, wherein the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).
 20. The golf ball according to claim 8, wherein the rubber composition includes the α,β-unsaturated carboxylic acid (b1), and the rubber composition further includes a metal compound (f).
 21. A golf ball comprising a core, a first cover positioned outside the core, a second cover positioned outside the first cover, a third cover positioned outside the second cover, and a fourth cover positioned outside the third cover, wherein when distances (%) from a central point of the core to nine points and JIS-C hardnesses at the nine points, which nine points are obtained by dividing a region from the central point of the core to a surface of the core at intervals of 12.5% of a radius of the core, are plotted in a graph, R² of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95, a JIS-C hardness H1 of the first cover is greater than a JIS-C hardness Hs at the surface of the core, and a JIS-C hardness H4 of the fourth cover is greater than the JIS-C hardness H1 of the first cover.
 22. The golf ball according to claim 21, wherein a difference (H1−Hs) between the hardness H1 and the hardness Hs is equal to or greater than 1.0.
 23. The golf ball according to claim 21, wherein a difference (H4−H1) between the hardness H4 and the hardness H1 is equal to or greater than 3 but equal to or less than
 30. 24. The golf ball according to claim 21, wherein a difference (Hs−H(0)) between the hardness Hs and a JIS-C hardness H(0) at the central point of the core is equal to or greater than
 15. 25. The golf ball according to claim 21, wherein a thickness T1 of the first cover is equal to or less than 1.8 mm, a thickness T2 of the second cover is equal to or less than 1.8 mm, a thickness T3 of the third cover is equal to or less than 1.8 mm, and a thickness T4 of the fourth cover is equal to or less than 1.8 mm.
 26. The golf ball according to claim 21, wherein a sum (T1+T2+T3+T4) of a thickness T1 of the first cover, a thickness T2 of the second cover, a thickness T3 of the third cover, and a thickness T4 of the fourth cover is equal to or less than 6.0 mm.
 27. The golf ball according to claim 21, wherein the core is formed by a rubber composition being crosslinked, the rubber composition includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, and the co-crosslinking agent (b) is: (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and/or (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.
 28. The golf ball according to claim 27, wherein an amount of the acid and/or the salt (d) is equal to or greater than 0.5 parts by weight but equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber (a).
 29. The golf ball according to claim 27, wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1).
 30. The golf ball according to claim 29, wherein the carboxylic acid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof.
 31. The golf ball according to claim 29, wherein a carbon number of a carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is equal to or greater than 1 but equal to or less than
 30. 32. The golf ball according to claim 29, wherein the carboxylic acid and/or the salt thereof (d1) is a zinc salt of a carboxylic acid.
 33. The golf ball according to claim 32, wherein the zinc salt of the carboxylic acid is one or more members selected from the group consisting of zinc octoate, zinc laurate, zinc myristate, and zinc stearate.
 34. The golf ball according to claim 27, wherein the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a).
 35. The golf ball according to claim 27, wherein the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a).
 36. The golf ball according to claim 27, wherein the rubber composition further includes an organic sulfur compound (e).
 37. The golf ball according to claim 36, wherein the organic sulfur compound (e) is at least one member selected from the group consisting of thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.
 38. The golf ball according to claim 36, wherein the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).
 39. The golf ball according to claim 27, wherein the rubber composition includes the α,β-unsaturated carboxylic acid (b1), and the rubber composition further includes a metal compound (f).
 40. The golf ball according to claim 21, wherein a hardness H2 of the second cover is greater than the hardness H1, a hardness H3 of the third cover is greater than the hardness H2, and the hardness H4 is greater than the hardness H3.
 41. The golf ball according to claim 40, wherein a difference (H2−H1) between the hardness H2 and the hardness H1 is equal to or greater than
 2. 42. The golf ball according to claim 40, wherein a difference (H3−H2) between the hardness H3 and the hardness H2 is equal to or greater than
 2. 43. The golf ball according to claim 40, wherein a difference (H4−H3) between the hardness H4 and the hardness H3 is equal to or greater than
 2. 44. A golf ball comprising a core, an inner cover positioned outside the core, a mid cover positioned outside the inner cover, and an outer cover positioned outside the mid cover, wherein when distances (%) from a central point of the core to nine points and JIS-C hardnesses at the nine points, which nine points are obtained by dividing a region from the central point of the core to a surface of the core at intervals of 12.5% of a radius of the core, are plotted in a graph, R² of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95, a JIS-C hardness Hs at the surface of the core is greater than a JIS-C hardness Hi of the inner cover, and a JIS-C hardness Ho of the outer cover is greater than the JIS-C hardness Hi of the inner cover.
 45. The golf ball according to claim 44, wherein a difference (Hs−Hi) between the hardness Hs and the hardness Hi is equal to or greater than
 1. 46. The golf ball according to claim 44, wherein a difference (Ho−Hi) between the hardness Ho and the hardness Hi is equal to or greater than 5 but equal to or less than
 30. 47. The golf ball according to claim 44, wherein a difference (Hs−H(0)) between the hardness Hs and a JIS-C hardness H(0) at the central point of the core is equal to or greater than
 15. 48. The golf ball according to claim 44, wherein a thickness Ti of the inner cover is equal to or less than 1.2 mm, a thickness Tm of the mid cover is equal to or less than 1.2 mm, and a thickness To of the outer cover is equal to or less than 1.2 mm.
 49. The golf ball according to claim 44, wherein a sum (Ti+Tm+To) of a thickness Ti of the inner cover, a thickness Tm of the mid cover, and a thickness To of the outer cover is equal to or less than 2.5 mm.
 50. The golf ball according to claim 44, wherein the JIS-C hardness Ho of the outer cover is greater than a JIS-C hardness Hm of the mid cover.
 51. The golf ball according to claim 50, wherein a difference (Ho−Hm) between the hardness Ho and the hardness Hm is equal to or greater than
 2. 52. The golf ball according to claim 44, wherein the core is formed by a rubber composition being crosslinked, the rubber composition includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, and the co-crosslinking agent (b) is: (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and/or (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.
 53. The golf ball according to claim 52, wherein an amount of the acid and/or the salt (d) is equal to or greater than 0.5 parts by weight but equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber (a).
 54. The golf ball according to claim 52, wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1).
 55. The golf ball according to claim 54, wherein a carbon number of a carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is equal to or greater than 1 but equal to or less than
 30. 56. The golf ball according to claim 54, wherein the carboxylic acid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof.
 57. The golf ball according to claim 52, wherein the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a).
 58. The golf ball according to claim 52, wherein the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a).
 59. The golf ball according to claim 52, wherein the rubber composition further includes an organic sulfur compound (e).
 60. The golf ball according to claim 59, wherein the organic sulfur compound (e) is at least one member selected from the group consisting of thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.
 61. The golf ball according to claim 59, wherein the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).
 62. The golf ball according to claim 52, wherein the rubber composition includes the α,β-unsaturated carboxylic acid (b1), and the rubber composition further includes a metal compound (f).
 63. The golf ball according to claim 52, wherein the acid and/or the salt (d) is a zinc salt of a carboxylic acid.
 64. The golf ball according to claim 63, wherein the zinc salt of the carboxylic acid is one or more members selected from the group consisting of zinc octoate, zinc laurate, zinc myristate, and zinc stearate.
 65. A golf ball comprising a core, an inner cover positioned outside the core, a first mid cover positioned outside the inner cover, a second mid cover positioned outside the first mid cover, and an outer cover positioned outside the second mid cover, wherein when distances (%) from a central point of the core to nine points and JIS-C hardnesses at the nine points, which nine points are obtained by dividing a region from the central point of the core to a surface of the core at intervals of 12.5% of a radius of the core, are plotted in a graph, R² of a linear approximation curve obtained by a least-square method is equal to or greater than 0.95, a JIS-C hardness Hs at the surface of the core is greater than a JIS-C hardness Hi of the inner cover, and a JIS-C hardness Ho of the outer cover is greater than the JIS-C hardness Hi of the inner cover.
 66. The golf ball according to claim 65, wherein a difference (Hs−Hi) between the hardness Hs and the hardness Hi is equal to or greater than
 1. 67. The golf ball according to claim 65, wherein a difference (Ho−Hi) between the hardness Ho and the hardness Hi is equal to or greater than 5 but equal to or less than
 30. 68. The golf ball according to claim 65, wherein the hardness Hs is greater than a JIS-C hardness H(0) at the central point of the core.
 69. The golf ball according to claim 68, wherein a difference (Hs−H(0)) between the hardness Hs and the hardness H(0) is equal to or greater than
 15. 70. The golf ball according to claim 65, wherein a thickness Ti of the inner cover is equal to or less than 1.8 mm, a thickness Tm1 of the first mid cover is equal to or less than 1.8 mm, a thickness Tm2 of the second mid cover is equal to or less than 1.8 mm, and a thickness To of the outer cover is equal to or less than 1.8 mm.
 71. The golf ball according to claim 70, wherein a sum (Ti+Tm1+Tm2+To) of the thickness Ti of the inner cover, the thickness Tm1 of the first mid cover, the thickness Tm2 of the second mid cover, and the thickness To of the outer cover is equal to or less than 6.0 mm.
 72. The golf ball according to claim 65, wherein the core is formed by a rubber composition being crosslinked, the rubber composition includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, and the co-crosslinking agent (b) is: (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and/or (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.
 73. The golf ball according to claim 72, wherein an amount of the acid and/or the salt (d) is equal to or greater than 1 parts by weight but less than 40 parts by weight, per 100 parts by weight of the base rubber (a).
 74. The golf ball according to claim 72, wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1).
 75. The golf ball according to claim 74, wherein a carbon number of a carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is equal to or greater than 1 but equal to or less than
 30. 76. The golf ball according to claim 74, wherein the carboxylic acid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof.
 77. The golf ball according to claim 72, wherein the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a).
 78. The golf ball according to claim 72, wherein the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a).
 79. The golf ball according to claim 72, wherein the rubber composition further includes an organic sulfur compound (e).
 80. The golf ball according to claim 79, wherein the organic sulfur compound (e) is at least one member selected from the group consisting of thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.
 81. The golf ball according to claim 79, wherein the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).
 82. The golf ball according to claim 72, wherein the rubber composition includes the α,β-unsaturated carboxylic acid (b1), and the rubber composition further includes a metal compound (f).
 83. The golf ball according to claim 72, wherein the acid and/or the salt (d) is a zinc salt of a carboxylic acid.
 84. The golf ball according to claim 83, wherein the zinc salt of the carboxylic acid is one or more members selected from the group consisting of zinc octoate, zinc laurate, zinc myristate, and zinc stearate. 